Merge pull request #74 from elmindreda/master

Replaced hardcoded GLFW link flags with pkgconfig
Include GLFW_INCLUDE_GLCOREARB only on Macs
Improved nvgTextGlyphPositions()

- added glyph min/max x
- fixed first glyph position
Removed GLFW 2 option macro.

GLFW_NO_GLU is a GLFW 2 feature.  GLFW 3 does not include the GLU header
by default.