meson: ensure correct version-def.h is used

To build the libgit-version library, Meson first generates
`version-def.h` in the build directory. Then it compiles `version.c`
into a library. During compilation, Meson tells to include both the
build directory and the project root directory.

However, when the user previously has compiled Git using Make, they will
have a `version-def.h` file in project root directory as well. Because
`version-def.h` is included in `version.c` using the #include directive
with double quotes, some preprocessors will look for the header file in
the same directory as the source file. This will cause compilation of
`version.c` ran by Meson to include `version-def.h` previously made by
Make, which might be out of date.

To explicitly tell the preprocessor which `version-def.h` to use, pass
the absolute path of this file as macro GIT_VERSION_H to the
preprocessor using option `-D` and have `version.c` `#include
GIT_VERSION_H`. To remain working with other build systems than Meson,
include "version-def.h" if that macro is not defined.

Co-authored-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

meson.build

@@ -1493,7 +1493,9 @@ libgit_version_library = static_library('git-version',
     'version.c',
     version_def_h,
   ],
-  c_args: libgit_c_args,
+  c_args: libgit_c_args + [
+    '-DGIT_VERSION_H="' + version_def_h.full_path() + '"',
+  ],
   dependencies: libgit_dependencies,
   include_directories: libgit_include_directories,
 )

version.c

@@ -1,8 +1,13 @@
 #include "git-compat-util.h"
 #include "version.h"
-#include "version-def.h"
 #include "strbuf.h"
 
+#ifndef GIT_VERSION_H
+# include "version-def.h"
+#else
+# include GIT_VERSION_H
+#endif
+
 const char git_version_string[] = GIT_VERSION;
 const char git_built_from_commit_string[] = GIT_BUILT_FROM_COMMIT;