For parallel builds, avoid multiple execution of vmodtool

Otherwise builds can fail due to partially written files from vmodtool
running on another target.

src/Makefile.am

@@ -24,7 +24,15 @@ libvmod_re2_la_LIBADD = @RE2_LIBS@
 
 vmod_re2.lo: vcc_if.c vcc_if.h
 
-vcc_if.c: vcc_if.h
+vmod_re2.lo: $(nodist_libvmod_re2_la_SOURCES)
+
+vcc_if.h vmod_re2.rst vmod_re2.man.rst: vcc_if.c
+
+vcc_if.c: vmod_re2.vcc
+	$(AM_V_VMODTOOL) $(PYTHON) $(VMODTOOL) -o vcc_if $(srcdir)/vmod_re2.vcc
+
+vmod_re2.3: vmod_re2.man.rst
+	$(AM_V_GEN) $(RST2MAN) vmod_re2.man.rst vmod_re2.3
 
 vre2/vre2.cpp: vre2/vre2.h