Small compilation bug fix and new API calls proposal

2 commits related to the compilation phase:
- pkg.m4 (missing on mingw 4.6.x nor 4.8.x and mac 10.9.4 pkg-config) for PKG_CHECK_MODULES macro. (d2de969)
- Dynamic library not being created with mingw gcc 4.6.x nor 4.8.x. (c5ea9ae)

Another commit (2f32cf2) for evaluation, related to decoupling and simplifying the library usage as shown in demo_hook.c by moving platform dependent functions to each already existent platform dependent implementation.
Commit 9b59d6a is just a comment on the demo_hook.