Fix linking for not-so-clever linkers.

On one of my systems, the linker is not intelligent enough to link with
pager.o (in libgit.a) when only the variable pager_in_use is needed. The
consequence is that the linker complains about an undefined variable. So,
put the variable into environment.o, where it is linked always.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

environment.c

@@ -21,6 +21,7 @@ char git_commit_encoding[MAX_ENCODING_LENGTH] = "utf-8";
 int shared_repository = PERM_UMASK;
 const char *apply_default_whitespace = NULL;
 int zlib_compression_level = Z_DEFAULT_COMPRESSION;
+int pager_in_use;
 
 static char *git_dir, *git_object_dir, *git_index_file, *git_refs_dir,
 	*git_graft_file;

pager.c

@@ -5,8 +5,6 @@
  * something different on Windows, for example.
  */
 
-int pager_in_use;
-
 static void run_pager(const char *pager)
 {
 	execlp(pager, pager, NULL);