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

native_client_sdk/src/tools/common.mk

Index: native_client_sdk/src/tools/common.mk
diff --git a/native_client_sdk/src/tools/common.mk b/native_client_sdk/src/tools/common.mk
index 1a340fb72bf38ed3e896ad6bca33184461e90df8..909fcf6f674ccc22e02af421e960ab38449e13b5 100644
--- a/native_client_sdk/src/tools/common.mk
+++ b/native_client_sdk/src/tools/common.mk
@@ -69,6 +69,7 @@ CP:=python $(NACL_SDK_ROOT)/tools/oshelpers.py cp
 MKDIR:=python $(NACL_SDK_ROOT)/tools/oshelpers.py mkdir
 MV:=python $(NACL_SDK_ROOT)/tools/oshelpers.py mv
 RM:=python $(NACL_SDK_ROOT)/tools/oshelpers.py rm
+WHICH:=python $(NACL_SDK_ROOT)/tools/oshelpers.py which
 
 
 #
@@ -107,8 +108,25 @@ endef
 # The target for all versions
 #
 USABLE_TOOLCHAINS=$(filter $(OSNAME) newlib glibc pnacl,$(VALID_TOOLCHAINS))
+
+ifeq (1,$(NO_HOST_BUILDS))
+USABLE_TOOLCHAINS:=$(filter-out $(OSNAME),$(USABLE_TOOLCHAINS))

[noelallen_use_chromium, 2013/02/09 00:55:57]

$(warning Skipping host builds)

+endif
+
+.PHONY: CHECK_FOR_CL_EXE
+CHECK_FOR_CL_EXE:
+ifneq (,$(findstring win,$(USABLE_TOOLCHAINS)))
+ifeq (,$(findstring cl.exe,$(shell $(WHICH) cl.exe)))
+	$(warning To skip the host build use:)
+	$(warning "make all_versions NO_HOST_BUILDS=1")
+	$(error Unable to find cl.exe in PATH while building Windows host build)
+endif
+endif

[noelallen_use_chromium, 2013/02/09 00:55:57]

Won't this generate an empty target warning/error for non windows?

+
 $(foreach tool,$(USABLE_TOOLCHAINS),$(eval $(call TOOLCHAIN_RULE,$(tool),$(dep))))
-all_versions: $(TOOLCHAIN_LIST)
+
+.PHONY: all_versions
+all_versions: CHECK_FOR_CL_EXE $(TOOLCHAIN_LIST)
 
 #
 # Target to remove temporary files