building: Arch Linux prerequisites; fixes in build procedure

Arch Linux prerequisites provide dependecies for Arch Linux and steps
to install them.

Compiler suite changed from g++ and clang++ to gnu and llvm
respectively because of these errors in configure:

configure: error: unknown compiler suite: clang++. manually specify
--c-compiler and --cpp-compiler, or use one of [llvm, gnu]

configure: error: unknown compiler suite: g++. manually specify
--c-compiler and --cpp-compiler, or use one of [llvm, gnu]

Fix path to assets. Had to use absolute value path or environment
variable with absolute value path because make was failing with this
error:

you need to specify AGE2DIR (e.g. /home/user/.wine/drive_c/age).
Makefile:37: recipe for target 'media' failed
make: *** [media] Error 1
Merge pull request #115 from qlonik/fork

building: Prereq steps for Arch and fixes for build procedure