clang: warn when the comma operator is used

When compiling Git using `clang`, the `-Wcomma` option can be used to
warn about code using the comma operator (because it is typically
unintentional and wants to use the semicolon instead).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

config.mak.dev

@@ -40,6 +40,10 @@ DEVELOPER_CFLAGS += -Wvla
 DEVELOPER_CFLAGS += -Wwrite-strings
 DEVELOPER_CFLAGS += -fno-common
 
+ifneq ($(filter clang9,$(COMPILER_FEATURES)),)
+DEVELOPER_CFLAGS += -Wcomma
+endif
+
 ifneq ($(filter clang4,$(COMPILER_FEATURES)),)
 DEVELOPER_CFLAGS += -Wtautological-constant-out-of-range-compare
 endif