fix clean target for example makefiles if the compiler is not in PATH

examples/stm32/f4/Makefile.include

@@ -27,14 +27,18 @@ OBJCOPY		= $(PREFIX)-objcopy
 OBJDUMP		= $(PREFIX)-objdump
 GDB		= $(PREFIX)-gdb
 FLASH		= $(shell which st-flash)
+
+TOOLCHAIN_DIR ?= ../../../../..
 ifeq ($(wildcard ../../../../../lib/libopencm3_stm32f4.a),)
+ifneq ($(strip $(shell which $(CC))),)
 TOOLCHAIN_DIR := $(shell dirname `which $(CC)`)/../$(PREFIX)
+endif
 else
 ifeq ($(V),1)
 $(info We seem to be building the example in the source directory. Using local library!)
 endif
-TOOLCHAIN_DIR := ../../../../..
 endif
+
 CFLAGS		+= -Os -g -Wall -Wextra -I$(TOOLCHAIN_DIR)/include \
 		   -fno-common -mcpu=cortex-m4 -mthumb \
 		   -mfloat-abi=hard -mfpu=fpv4-sp-d16 -MD -DSTM32F4