auto merge of #12433 : alexcrichton/rust/fix-some-config-things, r=brson

These are mostly centered around using an external LLVM (notably 3.5)

configure

@@ -604,7 +604,7 @@ then
     LLVM_VERSION=$($LLVM_CONFIG --version)
 
     case $LLVM_VERSION in
-        (3.[2-4]svn|3.[2-4])
+        (3.[2-5]svn|3.[2-5])
             msg "found ok version of LLVM: $LLVM_VERSION"
             ;;
         (*)
@@ -626,7 +626,7 @@ then
                       | cut -d ' ' -f 2)
 
     case $CFG_CLANG_VERSION in
-        (3.0svn | 3.0 | 3.1* | 3.2* | 3.3* | 3.4* )
+        (3.0svn | 3.0 | 3.1* | 3.2* | 3.3* | 3.4* | 3.5* )
         step_msg "found ok version of CLANG: $CFG_CLANG_VERSION"
         CFG_C_COMPILER="clang"
         ;;

mk/main.mk

@@ -218,8 +218,12 @@ LLVM_TOOLS=bugpoint llc llvm-ar llvm-as llvm-dis llvm-mc opt llvm-extract
 define DEF_LLVM_VARS
 # The configure script defines these variables with the target triples
 # separated by Z. This defines new ones with the expected format.
+ifeq ($$(CFG_LLVM_ROOT),)
 CFG_LLVM_BUILD_DIR_$(1):=$$(CFG_LLVM_BUILD_DIR_$(subst -,_,$(1)))
 CFG_LLVM_INST_DIR_$(1):=$$(CFG_LLVM_INST_DIR_$(subst -,_,$(1)))
+else
+CFG_LLVM_INST_DIR_$(1):=$$(CFG_LLVM_ROOT)
+endif
 
 # Any rules that depend on LLVM should depend on LLVM_CONFIG
 LLVM_CONFIG_$(1):=$$(CFG_LLVM_INST_DIR_$(1))/bin/llvm-config$$(X_$(1))