m4: Process pkg.m4 through AC_CONFIG_FILES

Currently there are no substitutions needed in pkg.m4, but this will
allow autoconf values such as the package version to be included.

Makefile.am

@@ -48,7 +48,7 @@ endif
 
 # Various data files
 m4dir = $(datadir)/aclocal
-dist_m4_DATA = pkg.m4
+m4_DATA = pkg.m4
 dist_doc_DATA = pkg-config-guide.html
 dist_man_MANS = pkg-config.1
 EXTRA_DIST = README.win32

configure.ac

@@ -195,7 +195,7 @@ dnl g_win32_get_package_installation_directory_of_module().
 dnl
 dnl Pull in pkg-config macros to find external glib.
 dnl
-m4_include([pkg.m4])
+m4_include([pkg.m4.in])
 m4_define([glib_module], [glib-2.0 >= 2.16])
 AC_ARG_WITH([internal-glib],
   [AS_HELP_STRING([--with-internal-glib], [use internal glib])],
@@ -255,6 +255,7 @@ AM_CONDITIONAL([HOST_TOOL], [test "x$enable_host_tool" = xyes])
 
 AC_CONFIG_FILES([
 Makefile
+pkg.m4
 check/Makefile
 check/config.sh
 ])