Autotools: Remove unused DEBUG_CFLAGS variable (#12659)

This removes the unused DEBUG_CFLAGS variable from configure.ac. It has
been once set in the build files similarly to Zend.m4 but was then
removed and simplified.

CS synced and DEBUG_CFLAGS checked and appended with AS_VAR_* macros.

Author: petk

Zend/Zend.m4

@@ -190,7 +190,7 @@ AX_CHECK_COMPILE_FLAG([-Wformat-truncation], CFLAGS="-Wformat-truncation $CFLAGS
 AX_CHECK_COMPILE_FLAG([-Wstrict-prototypes], CFLAGS="-Wstrict-prototypes $CFLAGS", , [-Werror])
 AX_CHECK_COMPILE_FLAG([-fno-common], CFLAGS="-fno-common $CFLAGS", , [-Werror])
 
-test -n "$DEBUG_CFLAGS" && CFLAGS="$CFLAGS $DEBUG_CFLAGS"
+AS_VAR_IF([DEBUG_CFLAGS],,, [AS_VAR_APPEND([CFLAGS], [" $DEBUG_CFLAGS"])])
 
 if test "$ZEND_ZTS" = "yes"; then
   AC_DEFINE(ZTS,1,[ ])

configure.ac

@@ -938,8 +938,6 @@ if test "$PHP_CONFIG_FILE_SCAN_DIR" = "DEFAULT"; then
 fi
 AC_MSG_RESULT([$PHP_CONFIG_FILE_SCAN_DIR])
 
-test -n "$DEBUG_CFLAGS" && CFLAGS="$CFLAGS $DEBUG_CFLAGS"
-
 PHP_ARG_ENABLE([sigchild],
   [whether to enable PHP's own SIGCHLD handler],
   [AS_HELP_STRING([--enable-sigchild],