add llvm configure flags to enable/disable target OSes/Envs

Makefile.rules

Index: Makefile.rules
===================================================================
--- Makefile.rules	(revision 138335)
+++ Makefile.rules	(working copy)
@@ -318,6 +318,16 @@
   CPP.Defines += -D_GLIBCXX_DEBUG -DXDEBUG
 endif
 
+# Take the list of TARGET_OSES_TO_BUILD and generate preprocessor symbols
+# indicating that the target OS is enabled.
+CPP.Defines += $(patsubst %,-DLLVM_ENABLED_TARGET_OS_%,\
+	$(shell echo $(TARGET_OSES_TO_BUILD) | tr a-z A-Z))
+
+# Take the list of TARGET_ENVS_TO_BUILD and generate preprocessor symbols
+# indicating that the target environment is enabled.
+CPP.Defines += $(patsubst %,-DLLVM_ENABLED_TARGET_ENV_%,\
+	$(shell echo $(TARGET_ENVS_TO_BUILD) | tr a-z A-Z))
+
 # LOADABLE_MODULE implies several other things so we force them to be
 # defined/on.
 ifdef LOADABLE_MODULE