[C++] Use 4 spaces in CMakeLists.txt.
[C++] Use 4 spaces in CMakeLists.txt.

EvilMcJerkface/Aeron,mikeb01/Aeron,galderz/Aeron,galderz/Aeron,EvilMcJerkface/Aeron,mikeb01/Aeron,real-logic/Aeron,EvilMcJerkface/Aeron,galderz/Aeron,real-logic/Aeron,EvilMcJerkface/Aeron,mikeb01/Aeron,mikeb01/Aeron,galderz/Aeron,real-logic/Aeron,real-logic/Aeron