Allow swiftshader to be used as a subproject.

Allow swiftshader to be used as a subproject.

We basically replace all references to CMAKE_SOURCE_DIR and
CMAKE_BINARY_DIR with CMAKE_CURRENT_SOURCE_DIR and
CMAKE_CURRENT_BINARY_DIR.

This enables other cmake projects to take a direct dependency on this
project.

Test: cmake . still shows error for in source builds.
Test: mkdir build && cd build && cmake .. -G Ninja && ninja succeeds
Change-Id: Ibb8366f8e7fd3b3152634c71324a5182d269e647
Reviewed-on: https://swiftshader-review.googlesource.com/c/22692
Reviewed-by: Lingfeng Yang <lfy@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Erwin Jansen <jansene@google.com>
Tested-by: Erwin Jansen <jansene@google.com>