Extensions should have the final say on their compiler flags

Currently compiler flags passed by extensions using the standard
``PHP_NEW_EXTENSION`` and ``PHP_ADD_SOURCES`` m4 macros are prepended
before the ones defined by ``Zend/Zend.m4``.

This was not really an issue before as ``Zend.m4`` only included
``-Wall`` but since the addition of ``-Wextra`` various issue about
disabling flags have been brought up.

A preliminary attempt was done in commit 5c1cf76
but this turns out to be more or less irrelevant.

The root issue is that  ``PHP_NEW_EXTENSION`` and ``PHP_ADD_SOURCES`` call the
``PHP_ADD_SOURCES_X`` macro and pass their flags as the 3rd argument which prepends
the flags. There exists a 6th argument for this macro which appends them but from a
cursory look at https://heap.space/search?full=PHP_ADD_SOURCES_X&project=php-src
this is not used. Moreover, the comment describing this macro explicitly informs
that this macro should not be used directly.

As such we drop the 6th argument of ``PHP_ADD_SOURCES_X`` and move the `special-flags`
argument to be appended instead of prepended.

Closes GH-6204

Author: Girgias

UPGRADING.INTERNALS

@@ -440,6 +440,14 @@ PHP 8.0 INTERNALS UPGRADE NOTES
     2.  The PHP_CHECK_GCC_ARG() m4 macro has been removed in favor of
         AX_CHECK_COMPILE_FLAG().
 
+    3. The 6th argument of PHP_ADD_SOURCES_X has been removed.
+
+    4. The 'special-flags' (3rd) argument of PHP_ADD_SOURCES_X are
+       now appended instead of prepended to previous compiler flags.
+       This means compiler flags passed to PHP_NEW_EXTENSION and PHP_ADD_SOURCES
+       are now appended, this allows to disable compiler flags set by Zend/Zend.m4
+       (e.g. disable certain compiler flags enabled by -Wextra)
+
   c. Windows build system changes
 
     - The configuration option --enable-crt-debug has been removed. The VC

build/php.m4

@@ -218,7 +218,7 @@ ifelse($1,shared,[
 ])
 
 dnl
-dnl PHP_ADD_SOURCES_X(source-path, sources[, special-flags[, target-var[, shared[, special-post-flags]]]])
+dnl PHP_ADD_SOURCES_X(source-path, sources[, special-flags[, target-var[, shared]]])
 dnl
 dnl Additional to PHP_ADD_SOURCES (see above), this lets you set the name of the
 dnl array target-var directly, as well as whether shared objects will be built
@@ -251,10 +251,10 @@ dnl Append to the array which has been dynamically chosen at m4 time.
 
 dnl Choose the right compiler/flags/etc. for the source-file.
       case $ac_src in
-        *.c[)] ac_comp="$b_c_pre $3 $ac_inc $b_c_meta -c $ac_srcdir$ac_src -o $ac_bdir$ac_obj.$b_lo $6$b_c_post" ;;
-        *.s[)] ac_comp="$b_c_pre $3 $ac_inc $b_c_meta -c $ac_srcdir$ac_src -o $ac_bdir$ac_obj.$b_lo $6$b_c_post" ;;
-        *.S[)] ac_comp="$b_c_pre $3 $ac_inc $b_c_meta -c $ac_srcdir$ac_src -o $ac_bdir$ac_obj.$b_lo $6$b_c_post" ;;
-        *.cpp|*.cc|*.cxx[)] ac_comp="$b_cxx_pre $3 $ac_inc $b_cxx_meta -c $ac_srcdir$ac_src -o $ac_bdir$ac_obj.$b_lo $6$b_cxx_post" ;;
+        *.c[)] ac_comp="$b_c_pre $ac_inc $b_c_meta $3 -c $ac_srcdir$ac_src -o $ac_bdir$ac_obj.$b_lo $b_c_post" ;;
+        *.s[)] ac_comp="$b_c_pre $ac_inc $b_c_meta $3 -c $ac_srcdir$ac_src -o $ac_bdir$ac_obj.$b_lo $b_c_post" ;;
+        *.S[)] ac_comp="$b_c_pre $ac_inc $b_c_meta $3 -c $ac_srcdir$ac_src -o $ac_bdir$ac_obj.$b_lo $b_c_post" ;;
+        *.cpp|*.cc|*.cxx[)] ac_comp="$b_cxx_pre $ac_inc $b_cxx_meta $3 -c $ac_srcdir$ac_src -o $ac_bdir$ac_obj.$b_lo $b_cxx_post" ;;
       esac
 
 dnl Create a rule for the object/source combo.