Makefile: move "strip" assignment down from flags

Move the assignment of the "STRIP" variable down to where we're
setting variables with the names of other programs.

For consistency with those use "=" for the assignment instead of
"?=". I can't imagine why this would need to be different than the
rest, and 4dc00021f7 ("Makefile: add 'strip' target", 2006-01-12)
which added it doesn't provide an explanation.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Makefile b/Makefile
index 97e922c..c53727e 100644
--- a/Makefile
+++ b/Makefile
@@ -512,7 +512,6 @@
 LDFLAGS =
 ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
-STRIP ?= strip
 
 # Create as necessary, replace existing, make ranlib unneeded.
 ARFLAGS = rcs
@@ -576,6 +575,7 @@
 PTHREAD_LIBS = -lpthread
 PTHREAD_CFLAGS =
 GCOV = gcov
+STRIP = strip
 SPATCH = spatch
 
 export TCL_PATH TCLTK_PATH