Implement amalgamation build

Amalgamation build consists of two files: "sampgdk.h" and "sampgdk.c".
In order to use them one has to define SAMPGDK_AMALGAMATION and
add both files to his/her project.

At this point there remains a dependency on subhook, i.e. you can't
use amalgamation without it. Hopefully I'll fix this in future.