Add -fnested-functions compile flag, if needed.

In order to use nested functions on some versions of GCC (e.g. GCC on OS X),
the -fnested-functions flag must be passed to the compiler. This patch adds
detection logic to ./configure to add the flag if necessary. It also adds
a comment to utils.h as to why the nested function needs a prototype.

(closes issue ASTERISK-20399)
Reported by: David M. Lee
Review: https://reviewboard.asterisk.org/r/2102/
........

Merged revisions 373119 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373120 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configure.ac

@@ -1038,6 +1038,18 @@ else
 fi
 AC_SUBST(AST_NATIVE_ARCH)
 
+dnl Nested functions required for RAII implementation
+AC_MSG_CHECKING(for -fnested-functions)
+AC_COMPILE_IFELSE(
+	dnl Prototype needed due to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36774
+	[AC_LANG_PROGRAM([], [auto void foo(void); void foo(void) {}])],
+	AC_MSG_RESULT(no)
+	[AST_NESTED_FUNCTIONS=],
+	AC_MSG_RESULT(required)
+	[AST_NESTED_FUNCTIONS=-fnested-functions]
+)
+AC_SUBST(AST_NESTED_FUNCTIONS)
+
 AC_MSG_CHECKING(for sysinfo)
 AC_LINK_IFELSE(
         [AC_LANG_PROGRAM([#include <sys/sysinfo.h>],