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>

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);