fix usleep undeclaration under Linux and add OpenGL linker flag to Makefile

Hello there,

  Appreciate the effort to put OpenCV and Gstreamer support to PTAM, while trying to compile the source code under Fedora 20, got two small issues described in the title. This pull-request will get them fixed.

  Also, I add a `.gitignore` file to drop the compiled the files out of the source code files.

Lihang