Use portable method to obtain hg id's output.

src/Makefile.am

@@ -48,13 +48,16 @@ EXTRA_wget_SOURCES = mswindows.c
 LDADD = $(ALLOCA) $(LIBOBJS)
 
 # version.c's not really _phony_, but it does need to be generated each run.
-HG_ID = $(shell hg id 2>/dev/null | cut -f 1 -d ' ')
 .PHONY: version.c
-version.c:
+version.c: hg-id
 	printf '%s' 'const char *version_string = "@VERSION@' > $@
-	-[ "$(HG_ID)" ] && printf '%s' ' ($(HG_ID))' >> $@
+	-HG_ID=$$(cat hg-id); [ "$$HG_ID" ] && printf '%s' " ($${HG_ID})" >> $@
 	printf '%s\n' '";' >> $@
 
+.INTERMEDIATE: hg-id
+hg-id:
+	-( hg id | cut -f 1 -d ' ' ) 2>/dev/null > $@
+
 check_LIBRARIES = libunittest.a
 libunittest_a_SOURCES = $(wget_SOURCES) test.c test.h
 libunittest_a_CPPFLAGS = -DTESTING