Suggested post-commit by @dblaikie, thanks!
scenario where a manual override may be desirable is when using Visual Studio
2017's CMake integration, which would not be detected as an IDE otherwise.
-**LLVM_ENABLE_MODULES**
- Compile with C++ modules enabled.
+**LLVM_ENABLE_MODULES**:BOOL
+ Compile with `Clang Header Modules
+ <https://clang.llvm.org/docs/Modules.html>`_.
**LLVM_ENABLE_PIC**:BOOL
Add the ``-fPIC`` flag to the compiler command-line, if the compiler supports