toolchain/gnu.inc.mk: don't check version if CC is not installed

With b30efee a warning was introduced when using `make term` without
the proper toolchain installed (e.g. when using BUILD_IN_DOCKER, but
`term` outside of the docker). This removes this warning

makefiles/toolchain/gnu.inc.mk

@@ -23,7 +23,7 @@ endif
 _OBJDUMP         := $(or $(shell command -v $(PREFIX)objdump || command -v gobjdump),objdump)
 OBJDUMP   ?= $(_OBJDUMP)
 
-GCC_VERSION := $(shell $(CC) -dumpversion | cut -d . -f 1)
+GCC_VERSION := $(shell command -v $(CC) > /dev/null && $(CC) -dumpversion | cut -d . -f 1)
 
 # -fmacro-prefix-map requires GCC 8
 ifneq (8, $(firstword $(shell echo 8 $(GCC_VERSION) | tr ' ' '\n' | sort -n)))