Autotools: Sync LIBS and LDFLAGS handling (#14956)

- ZEND_EXTRA_LIBS holds all possible libraries appended during the
  checks related to Zend.m4, moved after the ZEND_INIT check to make it
  more clear, redundant double quotes removed;
- EXTRA_LIBS has already been populated above in the configure.ac
- redundant 'unset LIBS' removed

configure.ac

@@ -1312,6 +1312,9 @@ fi
 
 ZEND_INIT
 
+ZEND_EXTRA_LIBS=$LIBS
+unset LIBS
+
 PHP_ADD_INCLUDE([$abs_srcdir], [1])
 PHP_ADD_INCLUDE([$abs_srcdir/main], [1])
 PHP_ADD_INCLUDE([$abs_builddir], [1])
@@ -1321,13 +1324,9 @@ PHP_ADD_INCLUDE([$abs_builddir/Zend])
 PHP_ADD_INCLUDE([$abs_srcdir/Zend])
 PHP_ADD_INCLUDE([$abs_srcdir/TSRM])
 
-ZEND_EXTRA_LIBS="$LIBS"
-unset LIBS
-
 EXTRA_LDFLAGS="$EXTRA_LDFLAGS $LDFLAGS"
 EXTRA_LDFLAGS_PROGRAM="$EXTRA_LDFLAGS_PROGRAM $LDFLAGS"
-EXTRA_LIBS="$EXTRA_LIBS $LIBS"
-unset LIBS LDFLAGS
+unset LDFLAGS
 
 AC_ARG_PROGRAM