If CMake finds a python interpreter, use it

The FreeBSD package building cluster installs e.g. 'python2.7', but no
plain 'python' to avoid version-related issues.

CMake's FindPythonInterp locates an interpreter with such a name and
provides it in the PYTHON_EXECUTABLE variable.  Use that if it's set,
falling back to the original '/usr/bin/env python' otherwise.

This is a missing part of LLDB commit r207122.
Patch by Brooks Davis in FreeBSD ports commit r353052

llvm-svn: 208204
diff --git a/lldb/scripts/Python/build-swig-Python.sh b/lldb/scripts/Python/build-swig-Python.sh
index 402d085..5f9b574 100755
--- a/lldb/scripts/Python/build-swig-Python.sh
+++ b/lldb/scripts/Python/build-swig-Python.sh
@@ -20,6 +20,8 @@
 makefile_flag=$7
 dependency_flag=$8
 
+PYTHON=${PYTHON_EXECUTABLE:-/usr/bin/env python}
+
 if [ -n "$makefile_flag" -a "$makefile_flag" = "-m" ]
 then
     MakefileCalled=1
@@ -292,7 +294,7 @@
     fi
 fi
 
-python_version=`/usr/bin/env python --version 2>&1 | sed -e 's,Python ,,' -e 's,[.][0-9],,2' -e 's,[a-z][a-z][0-9],,'`
+python_version=`${PYTHON} --version 2>&1 | sed -e 's,Python ,,' -e 's,[.][0-9],,2' -e 's,[a-z][a-z][0-9],,'`
 
 if [ $MakefileCalled -eq 0 ]
 then
@@ -300,9 +302,9 @@
 else
     if [ -n "${PYTHON_INSTALL_DIR}" ]
     then
-        framework_python_dir=`/usr/bin/env python -c "from distutils.sysconfig import get_python_lib; print get_python_lib(True, False, \"${PYTHON_INSTALL_DIR}\");"`/lldb
+        framework_python_dir=`${PYTHON} -c "from distutils.sysconfig import get_python_lib; print get_python_lib(True, False, \"${PYTHON_INSTALL_DIR}\");"`/lldb
     else
-        framework_python_dir=`/usr/bin/env python -c "from distutils.sysconfig import get_python_lib; print get_python_lib(True, False);"`/lldb
+        framework_python_dir=`${PYTHON} -c "from distutils.sysconfig import get_python_lib; print get_python_lib(True, False);"`/lldb
     fi
 fi
 
@@ -353,7 +355,7 @@
 current_dir=`pwd`
 if [ -f "${current_dir}/modify-python-lldb.py" ]
 then
-    /usr/bin/env python ${current_dir}/modify-python-lldb.py ${CONFIG_BUILD_DIR}
+    ${PYTHON} ${current_dir}/modify-python-lldb.py ${CONFIG_BUILD_DIR}
 fi