Do not clean up files generated by lib/libvcc/generate.py

... for a simple make clean, in particular not vmod_abi.h.

Otherwise, building from a trunk dist tarball will give a NOGIT
VMOD_ABI_Version after a "make clean", while the correct version
is in place for a simple make after unpacking the tarball.

include/Makefile.am

@@ -119,10 +119,10 @@ $(GEN_H): vmod_abi.h
 GENERATED_H = vmod_abi.h $(GEN_H)
 
 BUILT_SOURCES = $(GENERATED_H)
-MAINTAINERCLEANFILES = vcs_version.h
+
+MAINTAINERCLEANFILES = $(GENERATED_H)
 
 CLEANFILES = \
-	$(GENERATED_H) \
 	vrt_test \
 	_vrt_test \
 	_vrt.c