Improve make dist to warn on building inside a git repository

Author: Daniel Lange

Makefile.am

@@ -509,10 +509,13 @@ cppcheck:
 
 dist-hook: $(top_distdir)/configure
 	@if grep 'pkg_m4_absent' '$(top_distdir)/configure'; then \
-	   echo 'configure is generated without pkg.m4. Please supply pkg.m4 and run ./autogen.sh to rebuild the configure script.'>&2; \
+	   echo 'ERROR: configure is generated without pkg.m4. Please supply pkg.m4 and run ./autogen.sh to rebuild the configure script.'>&2; \
 	   (exit 1); \
 	else :; \
 	fi
+	@if grep 'PACKAGE_VERSION.*-g' '$(top_distdir)/configure'; then \
+	  echo 'WARNING: You are building a dist from a git version. Better run make dist outside of a .git repo on a tagged release.'>&2; \
+	fi
 
 .PHONY: lcov