Clean up compatibility definitions.

This attempts to clean up the way various compatibility
functions are defined and used.

 - A new header file, git-compat-util.h, is introduced.  This
   looks at various NO_XXX and does necessary function name
   replacements, equivalent of -Dstrcasestr=gitstrcasestr in the
   Makefile.

 - Those function name replacements are removed from the Makefile.

 - Common features such as usage(), die(), xmalloc() are moved
   from cache.h to git-compat-util.h; cache.h includes
   git-compat-util.h itself.

Signed-off-by: Junio C Hamano <junkio@cox.net>

compat/mmap.c

@@ -2,7 +2,7 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <errno.h>
-#include "../cache.h"
+#include "../git-compat-util.h"
 
 void *gitfakemmap(void *start, size_t length, int prot , int flags, int fd, off_t offset)
 {

compat/strcasestr.c

@@ -1,5 +1,4 @@
-#include <string.h>
-#include <ctype.h>
+#include "../git-compat-util.h"
 
 char *gitstrcasestr(const char *haystack, const char *needle)
 {