Allow for globbing on Google Test sources

[martinmine]

This PR enables globbing on ROOT_ADD_GTEST. Instead of specifying a test in CMake as

ROOT_ADD_GTEST(myTest test1.cxx test2.cxx test3.cxx)

This PR enables specifying it as:

ROOT_ADD_GTEST(myTest test*.cxx)

[phsft-bot]

Starting build on gcc49/centos7, native/mac1012, gcc49/slc6, gcc62/slc6, native/ubuntu14 with flags -Dvc=OFF -Dimt=ON -Dccache=ON
How to customize builds

[martinmine]

@peremato just committed this in 161503c, closing.

[phsft-bot]

Build failed on mac1012/native.
See console output.

Warnings:

• JENKINS-19022: warning: possible memory leak due to Git plugin usage; see: https://wiki.jenkins-ci.org/display/JENKINS/Remove+Git+Plugin+BuildsByBranch+BuildData

Failing tests:

• projectroot.roottest.python.stl.roottest_python_stl_stl