[xharness] Build with MSBuild. (#2752)

Time to embrace C# 7.

Make.config

@@ -244,6 +244,7 @@ SYSTEM_SN=$(MONO_PREFIX)/bin/sn
 SYSTEM_MONO=$(MONO_PREFIX)/bin/mono
 SYSTEM_MONO32=$(MONO_PREFIX)/bin/mono32
 SYSTEM_XBUILD=$(MONO_PREFIX)/bin/xbuild
+SYSTEM_MSBUILD=$(MONO_PREFIX)/bin/msbuild
 SYSTEM_RESGEN=$(MONO_PREFIX)/bin/resgen
 
 PKG_CONFIG=$(MONO_PREFIX)/bin/pkg-config

mk/quiet.mk

@@ -80,3 +80,5 @@ MMP_VERBOSITY=-vvvv
 MTOUCH_VERBOSITY=-vvvv
 MDTOOL_VERBOSITY=-v -v -v -v
 endif
+MSBUILD_VERBOSITY=$(XBUILD_VERBOSITY)
+MSBUILD_VERBOSITY_QUIET=$(XBUILD_VERBOSITY_QUIET)

tests/Makefile

@@ -171,7 +171,7 @@ $(TOP)/tools/mtouch/SdkVersions.cs: $(TOP)/tools/mtouch/SdkVersions.cs.in
 	@touch $@
 
 xharness/xharness.exe: $(wildcard xharness/*.cs) xharness/xharness.csproj $(TOP)/tools/mtouch/SdkVersions.cs test.config test-system.config .stamp-src-project-files
-	$(Q_GEN) $(SYSTEM_XBUILD) $(XBUILD_VERBOSITY_QUIET) xharness/xharness.csproj
+	$(Q_GEN) unset MSBuildExtensionsPath && $(SYSTEM_MSBUILD) $(MSBUILD_VERBOSITY_QUIET) xharness/xharness.csproj
 
 killall:
 	@killall "iPhone Simulator" >/dev/null 2>&1 || true