CMake: fix add_subdirectory(SwiftShader) when CMAKE_BUILD_TYPE is set in parent project
CMake: fix add_subdirectory(SwiftShader) when CMAKE_BUILD_TYPE is set in parent project

If parent project sets CMAKE_BUILD_TYPE as a non-cache variable
(e.g. like https://github.com/google/amber does), CMake emits an error
because we're trying to set it as a cache variable.

Bug: b/145758253
Change-Id: Ie169c45dd11674be9e40610f5dd711b8fd5830f8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43952
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Tested-by: Antonio Maiorano <86d87a6550afb6b07035c9743fa181900589f7e0@google.com>

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