CMakeList: Remove the CLANG_VERSION_STRING check
CMakeList: Remove the CLANG_VERSION_STRING check

It appears to be undefined for me.
Let Kokoro decide if it is required.

Change-Id: I0052703552dd0727e6d3dc12f7947ce06faee5f6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29054
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

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