Meson: Fix building libwpe as a subproject (#77)

Honor also MESON_SUBDIR in the version script. When building as a
subproject, MESON_SOURCE_ROOT points to the root directory of the
top-level project, and MESON_SUBDIR will be a non-empty relative path
which resolves to the current subdirectory, which is valid also for
subprojects.

Author: aperezdc

scripts/version.py

@@ -11,7 +11,7 @@
 
 version = {}
 version_re = re.compile(r"^#define\s+WPE_([A-Z]+)_VERSION\s+(\d+)$")
-version_file = path.join(environ["MESON_SOURCE_ROOT"],
+version_file = path.join(environ["MESON_SOURCE_ROOT"], environ["MESON_SUBDIR"],
                          "include", "wpe", "libwpe-version.h")
 
 with open(version_file, "r") as f: