Bug 1340588 - enable clang-cl to generate depfiles directly, rather than using a wrapper; r=build-peer

We use a wrapper script when compiling with MSVC to parse the
/showIncludes output and thereby generate a Makefile dependency
fragment.  This fragment enables us to do correct and faster incremental
builds.  But the cost of invoking the wrapper script can be significant;
it's an extra process or two to launch for every single compilation.

Instead, let's have clang-cl generate the dependencies directly, which
should be somewhat faster.

config/config.mk

@@ -120,8 +120,11 @@ CONFIG_TOOLS	= $(MOZ_BUILD_ROOT)/config
 AUTOCONF_TOOLS	= $(MOZILLA_DIR)/build/autoconf
 
 ifdef _MSC_VER
+# clang-cl is smart enough to generate dependencies directly.
+ifndef CLANG_CL
 CC_WRAPPER ?= $(call py_action,cl)
 CXX_WRAPPER ?= $(call py_action,cl)
+endif # CLANG_CL
 endif # _MSC_VER
 
 CC := $(CC_WRAPPER) $(CC)

js/src/old-configure.in

@@ -1603,6 +1603,13 @@ MOZ_ARG_HEADER(Build dependencies)
 if test "$GNU_CC" -a "$GNU_CXX"; then
   _DEPEND_CFLAGS='-MD -MP -MF $(MDDEPDIR)/$(@F).pp'
 else
+  # clang-cl doesn't accept the normal -MD -MP -MF options that clang does, but
+  # the underlying cc1 binary understands how to generate dependency files.
+  # These options are based on analyzing what the normal clang driver sends to
+  # cc1 when given the "correct" dependency options.
+  if test -n "$CLANG_CL"; then
+   _DEPEND_CFLAGS='-Xclang -MP -Xclang -MG -Xclang -dependency-file -Xclang $(MDDEPDIR)/$(@F).pp -Xclang -MT -Xclang $@'
+  fi
   dnl Don't override this for MSVC
   if test -z "$_WIN32_MSVC"; then
     _USE_CPP_INCLUDE_FLAG=

old-configure.in

@@ -3856,6 +3856,13 @@ if test "$COMPILE_ENVIRONMENT"; then
 if test "$GNU_CC" -a "$GNU_CXX"; then
   _DEPEND_CFLAGS='-MD -MP -MF $(MDDEPDIR)/$(@F).pp'
 else
+  # clang-cl doesn't accept the normal -MD -MP -MF options that clang does, but
+  # the underlying cc1 binary understands how to generate dependency files.
+  # These options are based on analyzing what the normal clang driver sends to
+  # cc1 when given the "correct" dependency options.
+  if test -n "$CLANG_CL"; then
+   _DEPEND_CFLAGS='-Xclang -MP -Xclang -MG -Xclang -dependency-file -Xclang $(MDDEPDIR)/$(@F).pp -Xclang -MT -Xclang $@'
+  fi
   dnl Don't override this for MSVC
   if test -z "$_WIN32_MSVC"; then
     _USE_CPP_INCLUDE_FLAG=