Workaround for Roslyn crash: https://github.com/mono/mono/issues/7017

--- mcs/build/profiles/basic.make.orig	2018-07-12 00:08:27 UTC
+++ mcs/build/profiles/basic.make
@@ -47,7 +47,7 @@ LIBRARY_COMPILE = $(BOOT_COMPILE)
 #
 # Copy from rules.make because I don't know how to unset MCS_FLAGS
 #
-USE_MCS_FLAGS = /codepage:$(CODEPAGE) /nologo /noconfig /deterministic $(LOCAL_MCS_FLAGS) $(PLATFORM_MCS_FLAGS) $(PROFILE_MCS_FLAGS) $(MCS_FLAGS)
+USE_MCS_FLAGS = /codepage:$(CODEPAGE) /parallel- /nologo /noconfig /deterministic $(LOCAL_MCS_FLAGS) $(PLATFORM_MCS_FLAGS) $(PROFILE_MCS_FLAGS) $(MCS_FLAGS)

 .PHONY: profile-check do-profile-check
 profile-check:
--- mcs/build/rules.make.orig	2018-07-12 00:09:18.508196000 +0000
+++ mcs/build/rules.make	2018-07-12 00:12:13.875877000 +0000
@@ -34,7 +34,7 @@
 endif
 endif

-USE_MCS_FLAGS = /codepage:$(CODEPAGE) /nologo /noconfig /deterministic $(LOCAL_MCS_FLAGS) $(PLATFORM_MCS_FLAGS) $(PROFILE_MCS_FLAGS) $(MCS_FLAGS)
+USE_MCS_FLAGS = /codepage:$(CODEPAGE) /parallel- /nologo /noconfig /deterministic $(LOCAL_MCS_FLAGS) $(PLATFORM_MCS_FLAGS) $(PROFILE_MCS_FLAGS) $(MCS_FLAGS)
 USE_MBAS_FLAGS = /codepage:$(CODEPAGE) $(LOCAL_MBAS_FLAGS) $(PLATFORM_MBAS_FLAGS) $(PROFILE_MBAS_FLAGS) $(MBAS_FLAGS)
 USE_CFLAGS = $(LOCAL_CFLAGS) $(CFLAGS) $(CPPFLAGS)
 CSCOMPILE = $(Q_MCS) $(MCS) $(USE_MCS_FLAGS)