build: Mechanically generate ABI versions from version number

If we're strict about applying something resembling semantic versioning
to the "marketing" version number, then we can mechanically generate
the ABI version from it.

This limits the range of valid micro versions (patchlevels) to 0-99.

Signed-off-by: Simon McVittie <smcv@collabora.com>

configure.ac

@@ -14,10 +14,17 @@ dnl Set various version strings - taken gratefully from the GTk sources
 SDL_MAJOR_VERSION=2
 SDL_MINOR_VERSION=23
 SDL_MICRO_VERSION=0
-SDL_INTERFACE_AGE=0
-SDL_BINARY_AGE=23
 SDL_VERSION=$SDL_MAJOR_VERSION.$SDL_MINOR_VERSION.$SDL_MICRO_VERSION
 
+SDL_BINARY_AGE=`expr $SDL_MINOR_VERSION \* 100 + $SDL_MICRO_VERSION`
+AS_CASE(["$SDL_MINOR_VERSION"],
+  [*[02468]],
+    dnl Stable branch, 2.24.1 -> libSDL2-2.0.so.0.2400.1
+    [SDL_INTERFACE_AGE="$SDL_MICRO_VERSION"],
+  [*],
+    dnl Development branch, 2.23.1 -> libSDL2-2.0.so.0.2301.0
+    [SDL_INTERFACE_AGE=0])
+
 AC_SUBST(SDL_MAJOR_VERSION)
 AC_SUBST(SDL_MINOR_VERSION)
 AC_SUBST(SDL_MICRO_VERSION)
@@ -33,9 +40,13 @@ LT_LANG([Windows Resource])
 # version twice: libSDL2-2.0.so.0.
 # TODO: in SDL 3, stop using -release, which will simplify it to libSDL3.so.0
 LT_RELEASE=2.0
-LT_CURRENT=`expr $SDL_MINOR_VERSION - $SDL_INTERFACE_AGE`
-LT_REVISION=$SDL_INTERFACE_AGE
+# Increment this if there is an incompatible change - but if that happens,
+# we should rename the library from SDL2 to SDL3, at which point this would
+# reset to 0 anyway.
+LT_MAJOR=0
 LT_AGE=`expr $SDL_BINARY_AGE - $SDL_INTERFACE_AGE`
+LT_CURRENT=`expr $LT_MAJOR + $LT_AGE`
+LT_REVISION=$SDL_INTERFACE_AGE
 m4_pattern_allow([^LT_])
 
 AC_SUBST(LT_RELEASE)