try to avoid a make race

vcc_compile.h is generated and included by other compilation units, so
we need to make sure that this happens in sequence.

Seen on vtest:

make[5]: Entering directory
'/tmp/vtest.32_sun12.4/varnish-cache/lib/libvcc'
  CC       libvcc_a-vcc_acl.o
  CC       libvcc_a-vcc_action.o
  CC       libvcc_a-vcc_backend.o
  CC       libvcc_a-vcc_backend_util.o
  CC       libvcc_a-vcc_compile.o
  CC       libvcc_a-vcc_expr.o
mkdir -p ../../include/tbl
/opt/local/bin/python3.4 ../../lib/libvcc/generate.py \
    ../.. ../..
  CC       libvcc_a-vcc_parse.o
  CC       libvcc_a-vcc_storage.o
  CC       libvcc_a-vcc_symb.o
"vcc_compile.h", line 194: undefined symbol: VCL_RET_MAX
"vcc_compile.h", line 194: can not declare variably modified type at file scope
cc: acomp failed for vcc_parse.c
Makefile:668: recipe for target 'libvcc_a-vcc_parse.o' failed

lib/libvcc/Makefile.am

@@ -54,7 +54,7 @@ GEN_H = \
 	vcc_fixed_token.c \
 	vcc_token_defs.h
 
-$(GEN_H): vcc_obj.c
+$(GEN_H): vcc_obj.c vcc_compile.h
 
 GENERATED_H = vcc_obj.c $(GEN_H)