Build PHP extension with HHVM

Build FastStringSearch, Wikidiff2 and LuaSandbox with HHVM on Trusty
instances.

Creates:
    php-FastStringSearch-hhvm-build
    php-luasandbox-hhvm-build
    php-wikidiff2-hhvm-build

Bug: 63120
Change-Id: Ic7ca7b371e2542b52a8aecc65be2917928dfad6d
Workaround hphpize injecting some bad include path

After running cmake ., the CMakeLists.txt file has something like:

 set(HHVM_INCLUDE_DIRS;<snip>;/root/joe/hhvm)

That causes the build to abort with:

 cc1plus: error: /root/hhvm/joe/hhvm: Permission denied

The workaround for Jenkins is to use sed to strip the invalid path
entirely.

Bug: 68944
Change-Id: I126e135b75253e3b7fd75afe3737086ebcca4f00