Issue Description:

Gentoo has two Python versions installed.
Available Python interpreters:
[1] python2.7
[2] python3.4 *
If Python 3.4 is default version the build script does not run at all. The error I got:

utils/build-script --preset=buildbot_linux install_destdir=$PWD/swift_inst installable_package=$PWD/swift-gentoo.tar.gz
Traceback (most recent call last):
File "utils/build-script", line 2149, in <module>
sys.exit(main())
File "utils/build-script", line 2143, in main
return main_preset()
File "utils/build-script", line 1051, in main_preset
args.preset_substitutions, args.preset_file_names, args.preset)
File "/home/vedg/swift-source/swift/utils/SwiftBuildSupport.py", line 134, in get_preset_options
config = _load_preset_files_impl(preset_file_names, substitutions)
File "/home/vedg/swift-source/swift/utils/SwiftBuildSupport.py", line 72, in _load_preset_files_impl
if config.read(preset_file_names) == []:
File "/usr/lib64/python3.4/configparser.py", line 672, in read
self._read(fp, filename)
File "/usr/lib64/python3.4/configparser.py", line 1070, in _read
fpname, lineno)
configparser.DuplicateOptionError: While reading from '/home/vedg/swift-source/swift/utils/build-presets.ini' [line 328]: option 'mixin-preset' in section 'preset: buildbot_incremental,tools=RA,stdlib=RA,build' already exists

I did change the default Python to 2.7 using "eselect python set 1" command. Then it is working as expected but at the last installation stage I have got another error:

CMake Error at scripts/cmake_install.cmake:36 (file):
file INSTALL cannot find
"/home/vedg/swift-source/build/buildbot_linux/lldb-linux-x86_64/lib/python2.7".
Call Stack (most recent call first):
cmake_install.cmake:42 (include)

FAILED: cd /home/vedg/swift-source/build/buildbot_linux/lldb-linux-x86_64 && /usr/bin/cmake -P cmake_install.cmake
ninja: build stopped: subcommand failed.
utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting

Actually, "/home/vedg/swift-source/build/buildbot_linux/lldb-linux-x86_64/lib64/python3.4" has been created. So, it looks like Python 3.4 has been used for LLDB buid.

To continue I created a symlink from .../lib64/python3.4 to .../lib/python2.7
I know it is not good workaround but build process has been finished successfully.
After installation to /usr simple test:

swift
Welcome to Swift version 3.0-dev (LLVM 2f56c9717e, Clang a44b92f85b, Swift c487c38). Type :help for assistance.
1> 1+3
$R0: Int = 4
2>