build: Introduce LIBXML_MINOR_COMPAT

This is set to 0 for now but could be used to avoid ABI stability
issues.

CMakeLists.txt

@@ -403,6 +403,8 @@ if(CMAKE_C_COMPILER_ID MATCHES "Clang" OR CMAKE_C_COMPILER_ID STREQUAL "GNU")
     endif()
 endif()
 
+set(LIBXML_MINOR_COMPAT 0)
+math(EXPR LIBXML_SOVERSION "${LIBXML_MAJOR_VERSION} + ${LIBXML_MINOR_COMPAT}")
 set_target_properties(
     LibXml2
     PROPERTIES
@@ -411,7 +413,7 @@ set_target_properties(
     POSITION_INDEPENDENT_CODE ON
     PREFIX lib
     VERSION ${PROJECT_VERSION}
-    SOVERSION ${LIBXML_MAJOR_VERSION}
+    SOVERSION ${LIBXML_SOVERSION}
 )
 
 if(MSVC)

configure.ac

@@ -16,7 +16,12 @@ LIBXML_MINOR_VERSION=MINOR_VERSION
 LIBXML_MICRO_VERSION=MICRO_VERSION
 LIBXML_MICRO_VERSION_SUFFIX=
 LIBXML_VERSION=$LIBXML_MAJOR_VERSION.$LIBXML_MINOR_VERSION.$LIBXML_MICRO_VERSION$LIBXML_MICRO_VERSION_SUFFIX
-LIBXML_VERSION_INFO=`expr $LIBXML_MAJOR_VERSION + $LIBXML_MINOR_VERSION`:$LIBXML_MICRO_VERSION:$LIBXML_MINOR_VERSION
+
+# The oldest minor version we are compatible with.
+LIBXML_MINOR_COMPAT=0
+LIBXML_CURRENT=`expr $LIBXML_MAJOR_VERSION + $LIBXML_MINOR_VERSION`
+LIBXML_AGE=`expr $LIBXML_MINOR_VERSION - $LIBXML_MINOR_COMPAT`
+LIBXML_VERSION_INFO=$LIBXML_CURRENT:$LIBXML_MICRO_VERSION:$LIBXML_AGE
 
 LIBXML_VERSION_NUMBER=`expr $LIBXML_MAJOR_VERSION \* 10000 + $LIBXML_MINOR_VERSION \* 100 + $LIBXML_MICRO_VERSION`
 

meson.build

@@ -466,6 +466,7 @@ foreach file : xml_opt_src
     endif
 endforeach
 
+v_min_compat = 0
 xml_lib = library(
     'xml2',
     files(xml_src),
@@ -474,7 +475,7 @@ xml_lib = library(
     include_directories: config_dir,
     install: true,
     version: meson.project_version(),
-    soversion: v_maj,
+    soversion: v_maj + v_min_compat,
 )
 
 dep_inc = include_directories('include')