Makefile: emulate compile in $(HCO) target better

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> Signed-off-by: Junio C Hamano <gitster@pobox.com>