Fix -fno-delete-null-pointer-checks (#664)

Only use -fno-delete-null-pointer-checks if the supplied compiler knows
the flag. Clang does not understand the flag, and does not need it.

Makefile.include

@@ -14,7 +14,19 @@ endif
 # if you want to publish the board into the sources as an uppercase #define
 BB = $(shell echo $(BOARD)|tr 'a-z' 'A-Z')
 CPUDEF = $(shell echo $(CPU)|tr 'a-z' 'A-Z')
-CFLAGS += -DBOARD=$(BB) -DCPU_$(CPUDEF) -fno-delete-null-pointer-checks
+CFLAGS += -DBOARD=$(BB) -DCPU_$(CPUDEF)
+
+# Add `-fno-delete-null-pointer-checks` flag iff the compiler supports it.
+# GCC removes moves tests whether `x == NULL`, if previously `x` or even `x->y` was accessed.
+# 0x0 might be a sane memory location for embedded systems, so the test must not be removed.
+# Right now clang does not use the *delete-null-pointer* optimization, and does not understand the parameter.
+# Related issues: #628, #664.
+ifeq ($(shell $(CC) -fno-delete-null-pointer-checks -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
+ifeq ($(shell LANG=C $(CC) -fno-delete-null-pointer-checks -E - 2>&1 1>/dev/null </dev/null | grep warning: | grep -- -fno-delete-null-pointer-checks),)
+CFLAGS += -fno-delete-null-pointer-checks
+endif
+endif
+
 export CFLAGS
 
 export BINDIR =$(CURDIR)/bin/$(BOARD)/