Makefile: Add default warning flags

The flags are taken from the different warning levels of the build
system when they do not generate any error with the current upstreamed
platforms.

Change-Id: Ia70cff83bedefb6d2f0dd266394ef77fe47e7f65
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>

Makefile

@@ -193,23 +193,19 @@ ASFLAGS_aarch32		=	$(march32-directive)
 ASFLAGS_aarch64		=	$(march64-directive)
 
 WARNING1 := -Wextra
-WARNING1 += -Wunused -Wno-unused-parameter
 WARNING1 += -Wmissing-declarations
 WARNING1 += -Wmissing-format-attribute
 WARNING1 += -Wmissing-prototypes
 WARNING1 += -Wold-style-definition
-WARNING1 += -Wunused-but-set-variable
 WARNING1 += -Wunused-const-variable
 
 WARNING2 := -Waggregate-return
 WARNING2 += -Wcast-align
-WARNING2 += -Wdisabled-optimization
 WARNING2 += -Wnested-externs
 WARNING2 += -Wshadow
 WARNING2 += -Wlogical-op
 WARNING2 += -Wmissing-field-initializers
 WARNING2 += -Wsign-compare
-WARNING2 += -Wmaybe-uninitialized
 
 WARNING3 := -Wbad-function-cast
 WARNING3 += -Wcast-qual
@@ -219,8 +215,6 @@ WARNING3 += -Wpadded
 WARNING3 += -Wpointer-arith
 WARNING3 += -Wredundant-decls
 WARNING3 += -Wswitch-default
-WARNING3 += -Wpacked-bitfield-compat
-WARNING3 += -Wvla
 
 ifeq (${W},1)
 WARNINGS := $(WARNING1)
@@ -230,6 +224,16 @@ else ifeq (${W},3)
 WARNINGS := $(WARNING1) $(WARNING2) $(WARNING3)
 endif
 
+WARNINGS	+=		-Wunused -Wno-unused-parameter	\
+				-Wdisabled-optimization		\
+				-Wvla
+
+ifeq ($(findstring clang,$(notdir $(CC))),)
+WARNINGS	+=		-Wunused-but-set-variable	\
+				-Wmaybe-uninitialized		\
+				-Wpacked-bitfield-compat
+endif
+
 ifneq (${E},0)
 ERRORS := -Werror
 endif