Set the CMake default build type to Release.
Set the CMake default build type to Release.

CMake considers built-in configuration variables to be predefined, thus
setting CMAKE_BUILD_TYPE with a default value doesn't work because it
already exists. The FORCE attribute overrides it, but we have to guard
against overriding user-specified values.

Change-Id: Icf1cad6af15e788c5ae9ac8424f1ef0d29f9a1ff
Reviewed-on: https://swiftshader-review.googlesource.com/8377
Tested-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Reviewed-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>

google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader