Add compat/setenv.c, use in git.c.

There is no setenv() in Solaris 5.8.  The trivial calls to
setenv() were replaced by putenv() in a much earlier patch,
but setenv() was used again in git.c.  This patch just adds
a compat/setenv.c.

The rule for building git$(X) also needs to include compat.
objects and compiler flags.  Those are now in makefile vars
COMPAT_OBJS and COMPAT_CFLAGS.

Signed-off-by: E. Jason Riedy <ejr@cs.berkeley.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git.c

@@ -13,6 +13,10 @@
 # define PATH_MAX 4096
 #endif
 
+#ifdef NO_SETENV
+extern int gitsetenv(char *name, char *value, int overwrite);
+#endif
+
 static const char git_usage[] =
 	"Usage: git [--version] [--exec-path[=GIT_EXEC_PATH]] [--help] COMMAND [ ARGS ]";