A laundry list of things to clean up in the source base.

runtime_config.c
----------------
Move all getenv() checks to a central location under APIs for fetching
the option. getenv is a syscall and we want to avoid calling it as much
as possible. We should call it only once and then store the result to
a variable which is used in succeeding calls. 

A centralized location for all the runtime options of pocl would also 
allow more easier creation of a configuration file or another way of 
setting the options.

