build: Rename LIBMD_ABI to SOVERSION

This matches the semantics of the variable, and makes it independent of
the project, just as the package variables.

configure.ac

@@ -15,12 +15,12 @@ AM_INIT_AUTOMAKE(
 )
 AM_SILENT_RULES([yes])
 
-LIBMD_ABI_MAJOR=0
-LIBMD_ABI_MINOR=1
-LIBMD_ABI_PATCH=0
+SOVERSION_MAJOR=0
+SOVERSION_MINOR=1
+SOVERSION_PATCH=0
 
-LIBMD_ABI="$LIBMD_ABI_MAJOR:$LIBMD_ABI_MINOR:$LIBMD_ABI_PATCH"
-AC_SUBST([LIBMD_ABI])
+SOVERSION="$SOVERSION_MAJOR:$SOVERSION_MINOR:$SOVERSION_PATCH"
+AC_SUBST([SOVERSION])
 
 # Check and store if we got user supplied variables.
 user_CFLAGS=${CFLAGS-unset}

src/Makefile.am

@@ -31,7 +31,7 @@ EXTRA_libmd_la_DEPENDENCIES = \
 
 libmd_la_LDFLAGS = \
 	-no-undefined \
-	-version-number $(LIBMD_ABI) \
+	-version-number $(SOVERSION) \
 	# EOL
 if HAVE_LINKER_VERSION_SCRIPT
 libmd_la_LDFLAGS += \