Fix compile error with LLVM 3.6.

The llvm::Linker class now uses diagnostic messages.
Revert "Fix compile error with LLVM 3.6."

This reverts commit cc829485cc70dd74699f315534ae1634a75e78a5.
Merge of missing changes for 2.066.1.

Merge up to 0588458.
Allow overriding of header files.

This is the same behavior as DMD.
Merge pull request #752 from redstar/merge-2.066.1

Merge of missing changes for 2.066.1.
std.math: use most inline x86/x64 assembly only if real is defined as x87