build: Internally use a $(VSCTOOL) variable

And make vsc.am expect it as one of the input variables.

configure.ac

@@ -645,6 +645,7 @@ AC_SUBST(pkgsysconfdir)
 # VMOD variables
 AC_SUBST(vmoddir, [$\(pkglibdir\)/vmods])
 AC_SUBST(VMODTOOL, [$\(top_srcdir\)/lib/libvcc/vmodtool.py])
+AC_SUBST(VSCTOOL, [$\(top_srcdir\)/lib/libvsc/vsctool.py])
 
 # Check for linker script support
 gl_LD_VERSION_SCRIPT

vsc.am

 # Generic rule to generate C code from VSC files. VSC files must be listed
-# in the $(VSC_SRC) variable.
+# in the $(VSC_SRC) variable. The $(VSCTOOL) variable must point to the
+# location of vsctool.py, normally set up by varnish.m4 at configure time.
 
 VSC_GEN = $(VSC_SRC:.vsc=.c) $(VSC_SRC:.vsc=.h)
 
-$(VSC_GEN): $(top_srcdir)/lib/libvsc/vsctool.py
+$(VSC_GEN): $(VSCTOOL)
 
 .vsc.c:
-	$(AM_V_GEN) $(PYTHON) $(top_srcdir)/lib/libvsc/vsctool.py -ch $<
+	$(AM_V_GEN) $(PYTHON) $(VSCTOOL) -ch $<
 
 clean-local: vsc-clean