cmake: Don't override CMAKE_INSTALL_PREFIX

Stop forcing CMAKE_INSTALL_PREFIX="install". If the user manually set
CMAKE_INSTALL_PREFIX, then trust that he knows what he's doing.

This patch does NOT change the project's default value ("install") of
CMAKE_INSTALL_PREFIX.

Change-Id: I81b46dd1986427b498fe6316bed03f01689987d4

CMakeLists.txt

 cmake_minimum_required(VERSION 2.8)
 
-set(CMAKE_INSTALL_PREFIX "install" CACHE STRING "prefix" FORCE)
+set(CMAKE_INSTALL_PREFIX "install" CACHE STRING "prefix")
 
 project(glslang)