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

examples/lpc43xx/Makefile.include

@@ -27,14 +27,18 @@ LD		= $(PREFIX)-gcc
 OBJCOPY		= $(PREFIX)-objcopy
 OBJDUMP		= $(PREFIX)-objdump
 GDB		= $(PREFIX)-gdb
+
+TOOLCHAIN_DIR ?= ../../../..
 ifeq ($(wildcard ../../../../lib/libopencm3_lpc43xx.a),)
+ifneq ($(strip $(shell which $(CC))),)
 TOOLCHAIN_DIR := $(shell dirname `which $(CC)`)/../$(PREFIX)
+endif
 else
 ifeq ($(V),1)
 $(info We seem to be building the example in the source directory. Using local library!)
 endif
-TOOLCHAIN_DIR := ../../../..
 endif
+
 CFLAGS		+= -O2 -g -Wall -Wextra -I$(TOOLCHAIN_DIR)/include -fno-common \
 		   -mcpu=cortex-m4 -mthumb -MD \
 		   -mfloat-abi=hard -mfpu=fpv4-sp-d16