Changes in amdapp-2.7 on an AMD system

1. Current versions of x11-drivers/ati-drivers do not have
opencl use flag, but they still install opencl headers.
This results in dev-util/opencl-headers being dragged
by amdapp which in turn results in file collision.

2. dev-utl/amdapp-2.7 fails to install on a system with
x11-drivers/ati-drivers-12.8:

	Detected file collision(s):

	/usr/lib64/libaticalrt.so
	/usr/lib64/libamdocl64.so
	/usr/lib64/libaticalcl.so
	/usr/lib64/OpenCL/vendors/amd/libOpenCL.so.1
	/usr/lib32/libamdocl32.so
	/usr/lib32/OpenCL/vendors/amd/libOpenCL.so.1
	/usr/lib64/OpenCL/vendors/amd/libOpenCL.so
	/usr/lib32/OpenCL/vendors/amd/libOpenCL.so
	Searching all installed packages for file collisions...

	Press Ctrl-C to Stop

	x11-drivers/ati-drivers-12.8
	/usr/lib32/OpenCL/vendors/amd/libOpenCL.so
	/usr/lib32/OpenCL/vendors/amd/libOpenCL.so.1
	/usr/lib64/OpenCL/vendors/amd/libOpenCL.so
	/usr/lib64/OpenCL/vendors/amd/libOpenCL.so.1
	/usr/lib64/libaticalcl.so
	/usr/lib64/libaticalrt.so