[NaCl SDK] Always use cmd.exe for SHELL in Windows Makefiles

If SHELL is not set then make will search PATH for sh.exe. If it is found, it
will use that instead of cmd.exe. If the sh.exe exists in a path with a space,
the command will fail to run.

BUG=none
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/806283003

Cr-Commit-Position: refs/heads/master@{#309450}

native_client_sdk/src/resources/Makefile.index.template

@@ -7,6 +7,13 @@
 #   http://www.gnu.org/software/make/manual/make.html
 #
 
+# Always use cmd.exe as the shell on Windows. Otherwise Make may try to search
+# the path for sh.exe. If it is found in a path with a space, the command will
+# fail.
+ifeq ($(OS),Windows_NT)
+  SHELL := cmd.exe
+endif
+
 PROJECTS := \
 [[for project in projects:]]
   {{project}} \

native_client_sdk/src/tools/common.mk

@@ -142,7 +142,11 @@ endif
 # Disable DOS PATH warning when using Cygwin based NaCl tools on Windows.
 #
 ifeq ($(OSNAME),win)
-  CYGWIN?=nodosfilewarning
+  # Always use cmd.exe as the shell on Windows. Otherwise Make may try to
+  # search the path for sh.exe. If it is found in a path with a space, the
+  # command will fail.
+  SHELL := cmd.exe
+  CYGWIN ?= nodosfilewarning
   export CYGWIN
 endif