Autotools: Add PHP_EXTRA_CFLAGS environment variable

[petk]

This adds a new configure-phase environment variable PHP_EXTRA_CFLAGS which are appended to the CFLAGS in the Makefile rules. It sets the already present undocumented EXTRA_CFLAGS Makefile variable to not introduce issues with EXTRA_CFLAGS usage in the existing PECL extensions.

This enables setting configure-and-build-phase CFLAGS and build-phase PHP_EXTRA_CFLAGS separately to not affect configure checks:

./configure CFLAGS=... PHP_EXTRA_CFLAGS=...

Follow-up of #12050

[petk]

I'm still rechecking if this can be done differently a bit by modifying CFLAGS manually in the configure.ac directly. Because other Autotools based projects don't normally set such variable these days anymore. But Autoconf approach is otherwise to still set it separately from the CFLAGS... 🤔

[petk]

@iluuu1994 sorry for pinging you here. Do you remember perhaps again why would it be necessary to add extra compile options to the build through the variable? Is there still any case where using CFLAGS is cumbersome for that? Or was that issue only the override of the default Autoconf flags because that -g -O2 got removed doing that (for example, ./configure CFLAGS=-DFOOBAR=1). Basically, the EXTRA_CFLAGS shouldn't be necessary or they would somewhere?

[iluuu1994]

Or was that issue only the override of the default Autoconf flags because that -g -O2 got removed doing that (for example, ./configure CFLAGS=-DFOOBAR=1).

That was the primary one, but you also made this comment:
#12050 (comment)

The Clang+Cpp+Asan issue still exists. The build system will add the -fsanitize= flags not only to CFLAGS but also CXXFLAGS (as it should). However, Clang requires linking with clang++ when compiling C++ files with -fsanitize= for some reason. Do you reckon we can fix this? I'm not sure what the best approach would be.