Derive the library version from one place and give the .so a soname

The version was written down twice and agreed with itself by luck.
AKGL_VERSION was hand-maintained in include/akgl/game.h, and nothing
connected it to the build, which had no version at all:

- project() declared no VERSION, so the @PROJECT_VERSION@ substitution
  in akgl.pc.in expanded to nothing and every installed akgl.pc shipped
  an empty Version field. pkg-config --modversion returned "" and
  --atleast-version could not work at all.
- libakgl.so had no VERSION or SOVERSION, so there was no soname and no
  symlink chain. A stale libakgl could be paired with new headers
  silently, which is the failure libakerror added its own soname to
  prevent.
- MY_LIBRARY_VERSION at CMakeLists.txt:288 was never set anywhere. It
  built main_lib_dest as "lib/akgl-", and main_lib_dest was then never
  read. Dead line, removed.

project(akgl VERSION 0.1.0) is now the only place the number appears. It
drives the generated include/akgl/version.h, the shared library VERSION
and SOVERSION, and the Version field in akgl.pc. Held at 0.1.0 to match
the constant it replaces, so akgl_game_load_versioncmp still compares
savegames against the same value.

include/akgl/version.h is generated by configure_file from version.h.in
and publishes AKGL_VERSION, AKGL_VERSION_MAJOR/MINOR/PATCH and
AKGL_VERSION_AT_LEAST(major, minor, patch). That macro is the point:
libakstdlib could not write that test against libakerror, which
publishes no version macro, and had to feature-test on
AKERR_FIRST_CONSUMER_STATUS instead. game.h now includes the generated
header rather than defining the version itself, so consumers see
AKGL_VERSION exactly where they saw it before.

While the major version is 0 the soname carries major.minor, giving
libakgl.so.0.1. A plain SOVERSION 0 would assert that 0.1.0 and 0.2.0
are ABI-interchangeable, and they are not -- the preceding commit alone
added akgl_error_init. At 1.0.0 the soname becomes the major alone,
matching libakerror.

akgl_version() in src/version.c reports the version of the libakgl that
is actually linked, where AKGL_VERSION reports what the caller compiled
against. Comparing the two is the only way to catch a stale shared
library on the loader path; the macro alone cannot see it. It returns a
string rather than an akerr_ErrorContext * because it cannot fail, for
the same reason akerr_name_for_status does.

Add tests/version.c: the string and the numeric components must describe
one version, the linked library must agree with the headers, and
AKGL_VERSION_AT_LEAST must order correctly at the major, minor and patch
boundaries. Verified out of band that bumping project() to 1.2.3 moves
version.h, akgl.pc and the soname together, and that an install produces
the libakgl.so -> .so.0.1 -> .so.0.1.0 chain with SONAME libakgl.so.0.1.

Also ignore include/akgl/version.h. include/ precedes the build tree on
the include path, so a stray copy there would shadow the generated one
and pin every consumer to whatever version it was generated at.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

include/akgl/game.h

@@ -12,8 +12,9 @@
 #include "tilemap.h"
 #include "renderer.h"
 #include "physics.h"
-
-#define AKGL_VERSION                     "0.1.0"
+// AKGL_VERSION used to be defined here by hand, which is how akgl.pc came to
+// ship an empty Version field: nothing tied the two together.
+#include <akgl/version.h>
 
 #define AKGL_GAME_AUDIO_TRACK_BGM        1
 #define AKGL_GAME_AUDIO_MAX_TRACKS       64

include/akgl/version.h.in

@@ -0,0 +1,56 @@
+/**
+ * @file version.h
+ * @brief Declares the libakgl version, both as compiled against and as linked.
+ *
+ * GENERATED FILE -- edit include/akgl/version.h.in, never the copy in the build
+ * tree. Every value here comes from the project() call in CMakeLists.txt, which
+ * is also what sets the shared library's VERSION and SOVERSION and the Version
+ * field in akgl.pc. One number, one place, so the header, the soname and
+ * pkg-config cannot drift apart.
+ *
+ * AKGL_VERSION is the version you *compiled against*. akgl_version() reports the
+ * version of the libakgl you actually *linked*. They disagree when a stale
+ * shared library is ahead of the new one on the loader path -- the failure the
+ * soname exists to prevent and this pair exists to diagnose.
+ */
+
+#ifndef _AKGL_VERSION_H_
+#define _AKGL_VERSION_H_
+
+#define AKGL_VERSION               "@PROJECT_VERSION@"
+#define AKGL_VERSION_MAJOR         @PROJECT_VERSION_MAJOR@
+#define AKGL_VERSION_MINOR         @PROJECT_VERSION_MINOR@
+#define AKGL_VERSION_PATCH         @PROJECT_VERSION_PATCH@
+
+/**
+ * @brief True when the headers on the include path are at least the given version.
+ *
+ * For consumers that must build against more than one libakgl release. This is
+ * the test libakstdlib could not write against libakerror, which published no
+ * version macro and had to feature-test on AKERR_FIRST_CONSUMER_STATUS instead.
+ *
+ * @code
+ * #if AKGL_VERSION_AT_LEAST(0, 2, 0)
+ *     akgl_something_new();
+ * #endif
+ * @endcode
+ */
+#define AKGL_VERSION_AT_LEAST(major, minor, patch)			\
+    ((AKGL_VERSION_MAJOR > (major)) ||					\
+     (AKGL_VERSION_MAJOR == (major) && AKGL_VERSION_MINOR > (minor)) ||	\
+     (AKGL_VERSION_MAJOR == (major) && AKGL_VERSION_MINOR == (minor) && \
+      AKGL_VERSION_PATCH >= (patch)))
+
+/**
+ * @brief Report the version of the libakgl that is actually linked.
+ *
+ * Returns "major.minor.patch". Compare it against AKGL_VERSION to detect a
+ * stale shared library. Never returns NULL, and the storage is static -- the
+ * caller must not free or modify it.
+ *
+ * This returns a string rather than an akerr_ErrorContext * because it cannot
+ * fail, the same reason akerr_name_for_status() returns a name directly.
+ */
+const char *akgl_version(void);
+
+#endif // _AKGL_VERSION_H_