GDB pretty-printers for GLM.

Before:

(gdb) p pos
$1 = {{x = 5, r = 5, s = 5}, {y = 6, g = 6, t = 6}, {z = 7, b = 7, p = 7}}

After:

(gdb) p pos
$1 = ivec3(5, 6, 7)

User setup:

• install glm_pp.py to any directory
• add the contents of gdbinit to ~/.gdbinit, replacing .../glm_pp with that directory

System setup:

• # make install (installs to the system GDB Python directory)
• add python import glm_pp to ~/.gdbinit

Run make -C test for an example.