Fix a bunch of compile warnings

[bsiegert]

This PR fixes a number of warnings during the build with clang on macOS.

• Skip unknown compiler warning options
• Avoid per-iteration copies in some loops
• Delete a move constructor for a type containing a mutex
• Build up an exception string correctly

None of these changes are essential or high-priority but I hope you find them useful.

[depressed-pho]

Nice! Thank you for your fixes!