cpu/mips: use gnu99 by default

makefiles/arch/mips.inc.mk

@@ -32,7 +32,9 @@ endif
 # newlib commit 81c17949f0419d1c4fee421c60987ea1149522ae
 # https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=commitdiff;h=81c17949f0419d1c4fee421c60987ea1149522ae
 # Otherwise we get an error about a missing declaration of strnlen in some parts.
-export CFLAGS += -std=gnu99
+ifeq (, $(filter -std=%, $(CFLAGS)))
+  export CFLAGS += -std=gnu99
+endif
 export CFLAGS_CPU   = -EL -mabi=$(ABI)
 export CFLAGS_LINK  = -ffunction-sections -fno-builtin -fshort-enums -fdata-sections
 export CFLAGS_DBG   = -g3