Autotools: Use FPM_EXTRA_LIBS for systemd library

Instead of appending -lsystemd to global LIBS variable, this adds it as
needed only to FPM_EXTRA_LIBS as it was already used in the BUILD_FPM
invocation.

Follow-up of phpGH-14881

Author: petk

sapi/fpm/config.m4

@@ -424,7 +424,8 @@ if test "$PHP_FPM" != "no"; then
     [nobody],
     [no])
 
-  PHP_ARG_WITH([fpm-systemd],,
+  PHP_ARG_WITH([fpm-systemd],
+    [whether to enable systemd integration in PHP-FPM],
     [AS_HELP_STRING([--with-fpm-systemd],
       [Activate systemd integration])],
     [no],
@@ -448,24 +449,28 @@ if test "$PHP_FPM" != "no"; then
     [no],
     [no])
 
-  if test "$PHP_FPM_SYSTEMD" != "no" ; then
+  AS_VAR_IF([PHP_FPM_SYSTEMD], [no], [php_fpm_systemd=simple], [
     PKG_CHECK_MODULES([SYSTEMD], [libsystemd >= 209])
 
-    AC_DEFINE([HAVE_SYSTEMD], [1], [Whether FPM has systemd integration])
+    AC_DEFINE([HAVE_SYSTEMD], [1],
+      [Define to 1 if FPM has systemd integration.])
     PHP_FPM_SD_FILES="fpm/fpm_systemd.c"
-    PHP_EVAL_LIBLINE([$SYSTEMD_LIBS])
+
+    save_ext_shared=$ext_shared
+    ext_shared=yes
+    PHP_EVAL_LIBLINE([$SYSTEMD_LIBS], [FPM_EXTRA_LIBS])
     PHP_EVAL_INCLINE([$SYSTEMD_CFLAGS])
+    ext_shared=$save_ext_shared
+
     php_fpm_systemd=notify
 
     dnl Sanity check.
-    CFLAGS_save="$CFLAGS"
+    CFLAGS_save=$CFLAGS
     CFLAGS="$INCLUDES $CFLAGS"
     AC_CHECK_HEADER([systemd/sd-daemon.h],,
       [AC_MSG_ERROR([Required systemd/sd-daemon.h not found.])])
-    CFLAGS="$CFLAGS_save"
-  else
-    php_fpm_systemd=simple
-  fi
+    CFLAGS=$CFLAGS_save
+  ])
 
   if test "$PHP_FPM_ACL" != "no" ; then
     AC_CHECK_HEADERS([sys/acl.h])
@@ -611,4 +616,5 @@ if test "$PHP_FPM" != "no"; then
 
   PHP_SUBST([SAPI_FPM_PATH])
   PHP_SUBST([BUILD_FPM])
+  PHP_SUBST([FPM_EXTRA_LIBS])
 fi