Enable ifunc checks on FreeBSD 12+

This was disabled in 2019 due to problems reported in FreeBSD 11. The original
report (PHP bug 77284) includes a comment that FreeBSD 12 worked - which also
happens to be the first version ifunc use appeared in libc.

Close GH-12288

Author: Freaky

NEWS

@@ -5,6 +5,7 @@ PHP                                                                        NEWS
 Core:
   . Added zend_call_stack_get implementation for NetBSD, DragonFlyBSD,
     Solaris and Haiku. (David Carlier)
+  . Enabled ifunc checks on FreeBSD from the 12.x releases. (Freaky)
 
 Date:
   . Added DateTime[Immutable]::createFromTimestamp. (Marc Bennewitz)

configure.ac

@@ -562,10 +562,12 @@ dnl Checks for GCC function attributes on all systems except ones without glibc
 dnl Fix for these systems is already included in GCC 7, but not on GCC 6.
 dnl
 dnl At least some versions of FreeBSD seem to have buggy ifunc support, see
-dnl bug #77284. Conservatively don't use ifuncs on FreeBSD.
-AS_CASE([$host_alias], [*-*-*android*|*-*-*uclibc*|*-*-*musl*|*freebsd*|*openbsd*], [true], [
-  AX_GCC_FUNC_ATTRIBUTE([ifunc])
-  AX_GCC_FUNC_ATTRIBUTE([target])
+dnl bug #77284. Conservatively don't use ifuncs on FreeBSD prior to version 12.
+AS_CASE([$host_alias], [*-*-*android*|*-*-*uclibc*|*-*-*musl*|*openbsd*], [true], [
+  if test "`uname -s 2>/dev/null`" != "FreeBSD" || test "`uname -U 2>/dev/null`" -ge 1200000; then
+    AX_GCC_FUNC_ATTRIBUTE([ifunc])
+    AX_GCC_FUNC_ATTRIBUTE([target])
+  fi
 ])
 
 dnl Check for IPv6 support.