build: Add $(VSC_GEN) to $(BUILT_SOURCES)

This guarantees that VSC code is generated before compiling anything.

Fixes #3928

lib/libvsc/Makefile.am

@@ -19,4 +19,6 @@ VSC_SRC = \
 noinst_LTLIBRARIES = libvsc.la
 libvsc_la_SOURCES = $(VSC_SRC)
 
+BUILT_SOURCES = $(VSC_GEN)
+
 dist_pkgdata_SCRIPTS = vsctool.py

vmod/Makefile.am

@@ -41,6 +41,8 @@ VSC_SRC = VSC_debug.vsc
 
 libvmod_debug_la_SOURCES += $(VSC_SRC)
 
+BUILT_SOURCES = $(VSC_GEN)
+
 # Allow Vmod_wrong*_Data to be exported
 vmod_debug_symbols_regex = 'Vmod_.*_Data'
 

vsc.am

 # Generic rule to generate C code from VSC files. VSC files must be listed
 # 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.
+# The resulting $(VSC_GEN) variable must be added to $(BUILT_SOURCES).
 
 VSC_GEN = $(VSC_SRC:.vsc=.c) $(VSC_SRC:.vsc=.h)