CMake: [nfc] standardize 'if' style

CMake: [nfc] standardize 'if' style

The if-style used in CMake files had no consistency. Meanwhile, the expression
in a CMake `endif` is ignored. We learned in the process of constructing the
prior commit that this is supported right back to CMake 3.1. So this commit
standardizes on no-space, no-expression if-style.