glm: new package

OpenGL Mathematics (GLM) is a header only C++ mathematics library for
graphics software based on the OpenGL Shading Language (GLSL)
specification and released under the MIT license.

http://glm.g-truc.net/
gource: fix build

gource currently fails in the configure phase:

  configure: error: Could not link against -lGLU !

This is a very misleading error, it seems to happen because configure
doesn't find boost libraries and ends up with uninitialized variable(s).
That in turn cause it to fail later with this unrelated error.

Fix by using boost libraries, not only the headers. gource also grew a
dependency on GLM, so add that to buildInputs.