[PATCH 1/1] Makefile: only use compiler flags when supported

[Markus Mayer <mmayer@broadcom.com>]

We need to check if some compiler flags are supported as not all
compilers may do so. For instance, "-Wsuggest-attribute=format" is
supported by gcc but not by clang.

Using a compiler option that is not supported will most likely cause the
build to fail, since we are building with "-Werror" and the compiler will
issue a warning about the unknown option.

As a result, we only use the option if the compiler understands it.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
---
 Makefile | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index f1f0ab368cd3..febdca5742c5 100644
--- a/Makefile
+++ b/Makefile
@@ -3,6 +3,26 @@
 # Device Tree Compiler
 #
 
+# try-run and cc-option were stolen from the Linux kernel
+TMPOUT = .tmp_$$$$
+# try-run
+# Usage: option = $(call try-run, $(CC)...-o "$$TMP",option-ok,otherwise)
+# Exit code chooses option. "$$TMP" serves as a temporary file and is
+# automatically cleaned up.
+try-run = $(shell set -e;		\
+	TMP=$(TMPOUT)/tmp;		\
+	mkdir -p $(TMPOUT);		\
+	trap "rm -rf $(TMPOUT)" EXIT;	\
+	if ($(1)) >/dev/null 2>&1;	\
+	then echo "$(2)";		\
+	else echo "$(3)";		\
+	fi)
+
+# cc-option
+# Usage: cflags-y += $(call cc-option,-march=winchip-c6,-march=i586)
+cc-option = $(call try-run, \
+	$(CC) -Werror $(1) -c -x c /dev/null -o "$$TMP",$(1),$(2))
+
 #
 # Version information will be constructed in this order:
 # DTC_VERSION release version as MAJOR.MINOR.PATCH
@@ -18,9 +38,12 @@ CONFIG_LOCALVERSION =
 ASSUME_MASK ?= 0
 
 CPPFLAGS = -I libfdt -I . -DFDT_ASSUME_MASK=$(ASSUME_MASK)
+# Warnings where we need to check they are supported
+OPT_WARNINGS = $(call cc-option,-Wsuggest-attribute=format)
+# Regular warnings
 WARNINGS = -Wall -Wpointer-arith -Wcast-qual -Wnested-externs -Wsign-compare \
 	-Wstrict-prototypes -Wmissing-prototypes -Wredundant-decls -Wshadow \
-	-Wsuggest-attribute=format -Wwrite-strings
+	-Wwrite-strings $(OPT_WARNINGS)
 CFLAGS = -g -Os $(SHAREDLIB_CFLAGS) -Werror $(WARNINGS) $(EXTRA_CFLAGS)
 
 BISON = bison
-- 
2.46.0