Makefile: Move *_LIBS assignment into its own section

Now the only other non-program assignment in the previous list is
PTHREAD_CFLAGS, which'll be moved elsewhere in a follow-up change.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -572,7 +572,6 @@ TCLTK_PATH = wish
 XGETTEXT = xgettext
 MSGFMT = msgfmt
 CURL_CONFIG = curl-config
-PTHREAD_LIBS = -lpthread
 PTHREAD_CFLAGS =
 GCOV = gcov
 STRIP = strip
@@ -580,6 +579,9 @@ SPATCH = spatch
 
 export TCL_PATH TCLTK_PATH
 
+# Set our default LIBS variables
+PTHREAD_LIBS = -lpthread
+
 # user customisation variable for 'sparse' target
 SPARSE_FLAGS ?=
 # internal/platform customisation variable for 'sparse'