CMake: use directory variables consistently
CMake: use directory variables consistently

Bug: b/145758253
Change-Id: I15f02fc3f619aac398564f4aacfd30d1dadbe5ef
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43948
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Tested-by: Antonio Maiorano <86d87a6550afb6b07035c9743fa181900589f7e0@google.com>

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