[PATCH v2 4/4] Makefile: emulate compile in $(HCO) target better

[Denton Liu <liu.denton@gmail.com>]

Currently, when testing headers using `make hdr-check`, headers are
directly compiled. Although this seems to test the headers, this is too
strict since we treat the headers as C sources. As a result, this will
cause warnings to appear that would otherwise not, such as a static
variable definition intended for later use throwing a unused variable
warning.

In addition, on platforms that can run `make hdr-check` but require
custom flags, this target was failing because none of them were being
passed to the compiler. For example, on MacOS, the NO_OPENSSL flag was
being set but it was not being passed into compiler so the check was
failing.

Fix these problems by emulating the compile process better, including
test compiling dummy *.hcc C sources generated from the *.h files and
passing $(ALL_CFLAGS) into the compiler for the $(HCO) target so that
these custom flags can be used.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
---

Peff, thanks for the suggestion! I modified it a little bit so that we
wouldn't have to keep regenerating the *.hcc files unnecessarily.

I also considered piping into the compiler's stdin directly which I know
works for GCC (and _probably_ Clang) but I opted against it because I'm
not sure it's portable for other compilers. Maybe it's alright for this
to be less portable since it's a developer target?

 .gitignore |  1 +
 Makefile   | 12 +++++++++---
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/.gitignore b/.gitignore
index 521d8f4fb4..34efe125cb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -216,6 +216,7 @@
 /tags
 /TAGS
 /cscope*
+*.hcc
 *.obj
 *.lib
 *.res
diff --git a/Makefile b/Makefile
index f879697ea3..581cc617e3 100644
--- a/Makefile
+++ b/Makefile
@@ -1872,7 +1872,7 @@ ifndef V
 	QUIET_MSGFMT   = @echo '   ' MSGFMT $@;
 	QUIET_GCOV     = @echo '   ' GCOV $@;
 	QUIET_SP       = @echo '   ' SP $<;
-	QUIET_HDR      = @echo '   ' HDR $<;
+	QUIET_HDR      = @echo '   ' HDR $(<:hcc=h);
 	QUIET_RC       = @echo '   ' RC $@;
 	QUIET_SUBDIR0  = +@subdir=
 	QUIET_SUBDIR1  = ;$(NO_SUBDIR) echo '   ' SUBDIR $$subdir; \
@@ -2771,9 +2771,14 @@ ifndef GCRYPT_SHA256
 endif
 CHK_HDRS = $(filter-out $(EXCEPT_HDRS),$(patsubst ./%,%,$(LIB_H)))
 HCO = $(patsubst %.h,%.hco,$(CHK_HDRS))
+HCC = $(HCO:hco=hcc)
 
-$(HCO): %.hco: %.h FORCE
-	$(QUIET_HDR)$(CC) -include git-compat-util.h -I. -o /dev/null -c -xc $<
+%.hcc: %.h
+	@echo '#include "git-compat-util.h"' >$@
+	@echo '#include "$<"' >>$@
+
+$(HCO): %.hco: %.hcc FORCE
+	$(QUIET_HDR)$(CC) $(ALL_CFLAGS) -o /dev/null -c -xc $<
 
 .PHONY: hdr-check $(HCO)
 hdr-check: $(HCO)
@@ -3082,6 +3087,7 @@ clean: profile-clean coverage-clean cocciclean
 	$(RM) $(ALL_PROGRAMS) $(SCRIPT_LIB) $(BUILT_INS) git$X
 	$(RM) $(TEST_PROGRAMS)
 	$(RM) $(FUZZ_PROGRAMS)
+	$(RM) $(HCC)
 	$(RM) -r bin-wrappers $(dep_dirs)
 	$(RM) -r po/build/
 	$(RM) *.pyc *.pyo */*.pyc */*.pyo command-list.h $(ETAGS_TARGET) tags cscope*
-- 
2.23.0.248.g3a9dd8fb08