Change SOVERSION at API breaks

Since the SOVERSION doesn't need to follow the library's version and
simply needs to be monotonically increasing whenever we release
something that breaks the ABI, we can set some number and allow multiple
versions of the library to be installed side-by-side.

We start here with the minor version as that's what we release for now,
and it allows to backport this change to earlier versions.

CMakeLists.txt

@@ -127,6 +127,9 @@ STRING(REGEX REPLACE "^.*LIBGIT2_VERSION \"[0-9]+\\.([0-9]+).*$" "\\1" LIBGIT2_V
 STRING(REGEX REPLACE "^.*LIBGIT2_VERSION \"[0-9]+\\.[0-9]+\\.([0-9]+).*$" "\\1" LIBGIT2_VERSION_REV "${GIT2_HEADER}")
 SET(LIBGIT2_VERSION_STRING "${LIBGIT2_VERSION_MAJOR}.${LIBGIT2_VERSION_MINOR}.${LIBGIT2_VERSION_REV}")
 
+FILE(STRINGS "include/git2/version.h" GIT2_HEADER_SOVERSION REGEX "^#define LIBGIT2_SOVERSION [0-9]+$")
+STRING(REGEX REPLACE "^.*LIBGIT2_SOVERSION ([0-9]+)$" "\\1" LIBGIT2_SOVERSION "${GIT2_HEADER_SOVERSION}")
+
 # Find required dependencies
 INCLUDE_DIRECTORIES(src include)
 
@@ -397,7 +400,7 @@ MSVC_SPLIT_SOURCES(git2)
 
 IF (SONAME)
 	SET_TARGET_PROPERTIES(git2 PROPERTIES VERSION ${LIBGIT2_VERSION_STRING})
-	SET_TARGET_PROPERTIES(git2 PROPERTIES SOVERSION ${LIBGIT2_VERSION_MAJOR})
+	SET_TARGET_PROPERTIES(git2 PROPERTIES SOVERSION ${LIBGIT2_SOVERSION})
 	IF (LIBGIT2_FILENAME)
 		ADD_DEFINITIONS(-DLIBGIT2_FILENAME=\"${LIBGIT2_FILENAME}\")
 		SET_TARGET_PROPERTIES(git2 PROPERTIES OUTPUT_NAME ${LIBGIT2_FILENAME})

include/git2/version.h

@@ -12,4 +12,6 @@
 #define LIBGIT2_VER_MINOR 20
 #define LIBGIT2_VER_REVISION 0
 
+#define LIBGIT2_SOVERSION 20
+
 #endif