Explicitly set X to avoid potential build breakage

$X is appended to binary names for Windows builds (ie. git.exe).
Pollution from the environment can inadvertently trigger this behaviour,
resulting in 'git' turning into 'gitwhatever' without warning.

Signed-off-by: Michael Palimaka <kensington@astralcloak.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -428,6 +428,9 @@ PROGRAM_OBJS += upload-pack.o
 PROGRAM_OBJS += http-backend.o
 PROGRAM_OBJS += sh-i18n--envsubst.o
 
+# Binary suffix, set to .exe for Windows builds
+X =
+
 PROGRAMS += $(patsubst %.o,git-%$X,$(PROGRAM_OBJS))
 
 TEST_PROGRAMS_NEED_X += test-chmtime