This is a follow on ticket from WT-8418 with a set of small miscellaneous fixes and improvements, identified whilst integrating CMake with Evergreen (WT-8020). These fixes include:

• Extend CMake define_build_mode to accept libraries
• Unset HAVE_BUILD_MODE_X cache vars on error paths
• Fix ASan build mode flags for GCC builds
• Add a 'Coverage' build mode
• Separate compiler versions between C & CXX for toolchain files
• Detect if libclangfuzzer is available before building the fuzz test.