add "exit 1" to the rule testing libgap version output

Co-authored-by: Max Horn <max@quendi.de>

Makefile.rules

@@ -1277,6 +1277,7 @@ ifeq ($(shell PKG_CONFIG_PATH=$(DESTDIR)$(libdir)/pkgconfig pkg-config --modvers
  @echo "pkg-config reports correct version of libgap: "$(PACKAGE_VERSION)
 else
  @echo "pkg-config does not report version of libgap, or is not installed"
+ exit 1
 endif
 
 # test libgap's pkg-config's libgap flags fetching