Preliminary support for reporting build platform

Add preliminary support for detection of the build plaform, and reporting
of same with the `git version --build-options' command. This can be useful
for bug reporting, to distinguish between 32 and 64-bit builds for
example.

The current implementation can only distinguish between x86 and x86_64.
This will be extended in future patches. In addition, all 32-bit variants
(i686, i586, etc.) are collapsed into `x86'. An example of the output is:

   $ git version --build-options
   git version 2.9.3.windows.2.826.g06c0f2f
   sizeof-long: 4
   machine: x86_64

The label of `machine' was chosen so the new information will approximate
the output of `uname -m'.

Signed-off-by: Adric Norris <landstander668@gmail.com>

help.c

@@ -423,6 +423,8 @@ const char *help_unknown_cmd(const char *cmd)
 
 int cmd_version(int argc, const char **argv, const char *prefix)
 {
+	static char build_platform[] = GIT_BUILD_PLATFORM;
+
 	/*
 	 * The format of this string should be kept stable for compatibility
 	 * with external projects that rely on the output of "git version".
@@ -431,6 +433,7 @@ int cmd_version(int argc, const char **argv, const char *prefix)
 	while (*++argv) {
 		if (!strcmp(*argv, "--build-options")) {
 			printf("sizeof-long: %d\n", (int)sizeof(long));
+			printf("machine: %s\n", build_platform);
 			/* NEEDSWORK: also save and output GIT-BUILD_OPTIONS? */
 		}
 	}

help.h

@@ -33,3 +33,16 @@ extern void list_commands(unsigned int colopts, struct cmdnames *main_cmds, stru
  */
 extern void help_unknown_ref(const char *ref, const char *cmd, const char *error);
 #endif /* HELP_H */
+
+/*
+ * identify build platform
+ */
+#ifndef GIT_BUILD_PLATFORM
+	#if defined __x86__ || defined __i386__ || defined __i586__ || defined __i686__
+		#define GIT_BUILD_PLATFORM "x86";
+	#elif defined __x86_64__
+		#define GIT_BUILD_PLATFORM "x86_64";
+	#else
+		#define GIT_BUILD_PLATFORM "unknown";
+	#endif
+#endif