external/icu: Don't hardcode cl (so we can build with clang-cl instead)

Change-Id: I8e75b0ab2439592316fc0d871280a438e3ae2f1c

external/icu/UnpackedTarball_icu.mk

@@ -24,6 +24,7 @@ $(eval $(call gb_UnpackedTarball_add_patches,icu,\
 	external/icu/icu4c-icu11100.patch.1 \
 	external/icu/icu4c-scriptrun.patch \
 	external/icu/rtti.patch.0 \
+	external/icu/clang-cl.patch.0 \
 	$(if $(filter-out ANDROID,$(OS)),external/icu/icu4c-icudata-stdlibs.diff) \
 	$(if $(filter EMSCRIPTEN,$(OS)),external/icu/icu4c-emscripten.patch.1) \
 ))

external/icu/clang-cl.patch.0

+--- source/runConfigureICU
++++ source/runConfigureICU
+@@ -259,8 +259,8 @@
+     Cygwin/MSVC)
+         THE_OS="Windows with Cygwin"
+         THE_COMP="Microsoft Visual C++"
+-        CC=cl; export CC
+-        CXX=cl; export CXX
++        CC=${CC-cl}; export CC
++        CXX=${CXX-cl}; export CXX
+         RELEASE_CFLAGS='-Gy -MD'
+         RELEASE_CXXFLAGS='-Gy -MD'
+         DEBUG_CFLAGS='-Zi -MDd'