Remove obsolescent AC_C_INLINE Autoconf macro (#13429)

This macro defines the inline keyword to be `__inline__`, `__inline`,
or empty, based on the compiler inline support. Since PHP requires C99,
which has the inline keyword definition and all current compilers
support it, this check is redundant and not needed anymore.

Author: petk

Zend/Zend.m4

@@ -225,8 +225,6 @@ if test "$ZEND_ZTS" = "yes"; then
   CFLAGS="$CFLAGS -DZTS"
 fi
 
-AC_C_INLINE
-
 dnl Test and set the alignment define for ZEND_MM. This also does the
 dnl logarithmic test for ZEND_MM.
 AC_MSG_CHECKING(for MM alignment and log values)