git-compat-util.h: tweak the way _XOPEN_SOURCE is set on Solaris

On Solaris, when _XOPEN_EXTENDED is set, its header file forces the
programs to be XPG4v2, defeating any _XOPEN_SOURCE setting to say we are
XPG5 or XPG6.  Also on Solaris, XPG6 programs must be compiled with a c99
compiler, while non XPG6 programs must be compiled with a pre-c99 compiler.

So when compiling on Solaris, always refrain from setting _XOPEN_EXTENDED,
and then set _XOPEN_SOURCE to 600 or 500 based on whether a c99 compiler
is being used or not.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -39,7 +39,20 @@
 /* Approximation of the length of the decimal representation of this type. */
 #define decimal_length(x)	((int)(sizeof(x) * 2.56 + 0.5) + 1)
 
-#if !defined(__APPLE__) && !defined(__FreeBSD__)  && !defined(__USLC__) && !defined(_M_UNIX)
+#if defined(__sun__)
+ /*
+  * On Solaris, when _XOPEN_EXTENDED is set, its header file
+  * forces the programs to be XPG4v2, defeating any _XOPEN_SOURCE
+  * setting to say we are XPG5 or XPG6.  Also on Solaris,
+  * XPG6 programs must be compiled with a c99 compiler, while
+  * non XPG6 programs must be compiled with a pre-c99 compiler.
+  */
+# if __STDC_VERSION__ - 0 >= 199901L
+# define _XOPEN_SOURCE 600
+# else
+# define _XOPEN_SOURCE 500
+# endif
+#elif !defined(__APPLE__) && !defined(__FreeBSD__)  && !defined(__USLC__) && !defined(_M_UNIX)
 #define _XOPEN_SOURCE 600 /* glibc2 and AIX 5.3L need 500, OpenBSD needs 600 for S_ISLNK() */
 #define _XOPEN_SOURCE_EXTENDED 1 /* AIX 5.3L needs this */
 #endif