Merge branch 'ls/makefile-cflags-developer-tweak'

There is a new DEVELOPER knob that enables many compiler warning
options in the Makefile.

* ls/makefile-cflags-developer-tweak:
  add DEVELOPER makefile knob to check for acknowledged warnings

.travis.yml

@@ -21,12 +21,12 @@ addons:
 
 env:
   global:
+    - DEVELOPER=1
     - P4_VERSION="15.2"
     - GIT_LFS_VERSION="1.1.0"
     - DEFAULT_TEST_TARGET=prove
     - GIT_PROVE_OPTS="--timer --jobs 3 --state=failed,slow,save"
     - GIT_TEST_OPTS="--verbose --tee"
-    - CFLAGS="-g -O2 -Wall -Werror"
     - GIT_TEST_CLONE_2GB=YesPlease
     # t9810 occasionally fails on Travis CI OS X
     # t9816 occasionally fails with "TAP out of sequence errors" on Travis CI OS X

Documentation/CodingGuidelines

@@ -171,6 +171,11 @@ For C programs:
 
  - We try to keep to at most 80 characters per line.
 
+ - As a Git developer we assume you have a reasonably modern compiler
+   and we recommend you to enable the DEVELOPER makefile knob to
+   ensure your patch is clear of all compiler warnings we care about,
+   by e.g. "echo DEVELOPER=1 >>config.mak".
+
  - We try to support a wide range of C compilers to compile Git with,
    including old ones. That means that you should not use C99
    initializers, even if a lot of compilers grok it.

Makefile

@@ -380,6 +380,18 @@ ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
 STRIP ?= strip
 
+ifdef DEVELOPER
+CFLAGS += -Werror \
+	-Wdeclaration-after-statement \
+	-Wno-format-zero-length \
+	-Wold-style-definition \
+	-Woverflow \
+	-Wpointer-arith \
+	-Wstrict-prototypes \
+	-Wunused \
+	-Wvla
+endif
+
 # Create as necessary, replace existing, make ranlib unneeded.
 ARFLAGS = rcs