[m4-common] build: Remove dnl lines with m4 itself

[Philip Chimento <pchimento src gnome org>]

commit e6c481786b8d565b3d928c8a70fdc6cb82d3bf99
Author: Philip Chimento <philip chimento gmail com>
Date:   Sun Nov 13 15:43:03 2016 -0800

    build: Remove dnl lines with m4 itself
    
    Instead of a non-optimally-portable regex, use m4 itself to remove dnl
    lines. Plain m4 without any autoconf extensions will remove dnl, but not
    expand any of the autoconf stuff. This is because autoconf undefines all
    of the built-in m4 macros and defines new ones that plain m4 is unaware
    of.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=774299

 Makefile.am |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/Makefile.am b/Makefile.am
index 7ae8087..019631d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -26,8 +26,9 @@ dist_aclocal_DATA = \
        autoconf-archive/m4/ax_valgrind_check.m4                        \
        $(NULL)
 
+# Here, plain m4 is being used to strip 'dnl' comments.
 all-local: $(dist_aclocal_DATA)
-       cat $^ | grep -v '^#' | grep -v 'AX_PACKAGE_REQUIRES' | sed -e 's/\<dnl\>.*//' | grep -o 
'\<AX_[A-Z0-9_]*[A-Z0-9]\>' | sort | uniq > used
+       cat $^ | grep -v '^#' | grep -v 'AX_PACKAGE_REQUIRES' | m4 | grep -o '\<AX_[A-Z0-9_]*[A-Z0-9]\>' | 
sort | uniq > used
        cat $^ | grep ^AC_DEFUN | grep -o '\<AX_[A-Z0-9_]*\>' | sort | uniq > defined
        diff -u used defined
        rm -f used defined