[NaCl SDK] Print nice error if host build can't find gcc or cl.exe

native_client_sdk/src/tools/host_vc.mk

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 #
 # GNU Make based build file.  For details on GNU Make see:
 #   http://www.gnu.org/software/make/manual/make.html
 #
 #
 
 
 #
 # Macros for TOOLS
 #
 # We use the C++ compiler for everything and then use the -Wl,-as-needed flag
 # in the linker to drop libc++ unless it's actually needed.
 #
 HOST_CC?=cl.exe /nologo
 HOST_CXX?=cl.exe /nologo /EHsc
 HOST_LINK?=link.exe /nologo
 HOST_LIB?=lib.exe /nologo
 
+ifeq (,$(findstring cl.exe,$(shell $(WHICH) cl.exe)))
+$(warning To skip the host build use:)
+$(warning "make NO_HOST_BUILDS=1")
+$(error Unable to find cl.exe in PATH while building Windows host build)
+endif
+
 
 ifeq ('Debug','$(CONFIG)')
 WIN_OPT_FLAGS?=/Od /MTd /Z7
 else
 WIN_OPT_FLAGS?=/O2 /MT /Z7
 endif
 
 WIN_FLAGS?=-D WIN32 -D _WIN32 -D PTW32_STATIC_LIB
 
 
@@ skipping 88 matching lines @@
 NMF:=python $(NACL_SDK_ROOT)/tools/create_nmf.py
 
 define NMF_RULE
 NMF_LIST+=$(OUTDIR)/$(1).nmf
 $(OUTDIR)/$(1).nmf : $(OUTDIR)/$(1)$(HOST_EXT)
         @echo "Host Toolchain" > $$@
 endef
 
 all : $(LIB_LIST) $(DEPS_LIST) $(NMF_LIST)