Makefile: add $(DEVELOPER_CFLAGS) variable

This does not change the behavior, but allows the user to tweak
DEVELOPER_CFLAGS on the command-line or in a config.mak* file if
needed.

This also makes the code somewhat cleaner as it follows the pattern

<initialisation of variables>
<include statements>
<actual build logic>

by specifying which flags to activate in the first part, and actually
activating them in the last one.
Signed-off-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -375,6 +375,15 @@ GIT-VERSION-FILE: FORCE
 # CFLAGS and LDFLAGS are for the users to override from the command line.
 
 CFLAGS = -g -O2 -Wall
+DEVELOPER_CFLAGS = -Werror \
+	-Wdeclaration-after-statement \
+	-Wno-format-zero-length \
+	-Wold-style-definition \
+	-Woverflow \
+	-Wpointer-arith \
+	-Wstrict-prototypes \
+	-Wunused \
+	-Wvla
 LDFLAGS =
 ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
@@ -941,15 +950,7 @@ include config.mak.uname
 -include config.mak
 
 ifdef DEVELOPER
-CFLAGS += -Werror \
-	-Wdeclaration-after-statement \
-	-Wno-format-zero-length \
-	-Wold-style-definition \
-	-Woverflow \
-	-Wpointer-arith \
-	-Wstrict-prototypes \
-	-Wunused \
-	-Wvla
+CFLAGS += $(DEVELOPER_CFLAGS)
 endif
 
 ifndef sysconfdir