build: bring back configure warning when pkg.m4 is absent

Commit 24b1513 removed the warning
when the configure script is generated without pkg.m4. I added that
warning years ago (see 103f1a4) to
prevent downstream distributions from creating a tarball with an
"incomplete" configure script. Add the warning back, reword the
messages to tell exactly what feature would be missing (to builders),
and also add FORCE_MAKE_DIST variable for builders who want to ignore
the warning. :)

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>

Author: Explorer09

Makefile.am

@@ -508,8 +508,9 @@ cppcheck:
 	cppcheck -q -v . --enable=all -DHAVE_OPENVZ
 
 dist-hook: $(top_distdir)/configure
-	@if grep 'pkg_m4_absent' '$(top_distdir)/configure'; then \
-	   echo 'ERROR: configure is generated without pkg.m4. Please supply pkg.m4 and run ./autogen.sh to rebuild the configure script.'>&2; \
+	@if test "x$$FORCE_MAKE_DIST" = x && \
+	   grep 'pkg_m4_absent' '$(top_distdir)/configure' >/dev/null; then \
+	   echo 'ERROR: This distribution would have incomplete pkg-config support. Rebuilding the configure script is advised. Set FORCE_MAKE_DIST=1 to ignore this warning.'>&2; \
 	   (exit 1); \
 	else :; \
 	fi

configure.ac

@@ -384,6 +384,11 @@ dnl If the macro is not called, some pkg-config checks might be skipped
 dnl and $PKG_CONFIG might be unset.
 m4_ifdef([PKG_PROG_PKG_CONFIG], [
    PKG_PROG_PKG_CONFIG()
+], [
+   pkg_m4_absent=1 # Makefile might grep this keyword. Don't remove.
+   m4_warn(
+      [syntax],
+      [pkg.m4 is absent or older than version 0.16; this 'configure' would have incomplete pkg-config support])
 ])
 
 # HTOP_CHECK_SCRIPT(LIBNAME, FUNCTION, DEFINE, CONFIG_SCRIPT, ELSE_PART)