Rollup merge of #25971 - richo:configurable-python, r=alexcrichton

r? @alexcrichton

Author: Manishearth

configure

@@ -582,6 +582,7 @@ valopt sysconfdir "/etc" "install system configuration files"
 valopt datadir "${CFG_PREFIX}/share" "install data"
 valopt infodir "${CFG_PREFIX}/share/info" "install additional info"
 valopt llvm-root "" "set LLVM root"
+valopt python "" "set path to python"
 valopt jemalloc-root "" "set directory where libjemalloc_pic.a is located"
 valopt build "${DEFAULT_BUILD}" "GNUs ./configure syntax LLVM build triple"
 valopt android-cross-path "/opt/ndk_standalone" "Android NDK standalone path"
@@ -695,7 +696,9 @@ putvar CFG_BOOTSTRAP_KEY
 step_msg "looking for build programs"
 
 probe_need CFG_CURLORWGET  curl wget
-probe_need CFG_PYTHON      python2.7 python2.6 python2 python
+if [ -z "$CFG_PYTHON_PROVIDED" ]; then
+    probe_need CFG_PYTHON      python2.7 python2.6 python2 python
+fi
 
 python_version=$($CFG_PYTHON -V 2>&1)
 if [ $(echo $python_version | grep -c '^Python 2\.[4567]') -ne 1 ]; then