Minor mlib tweaks and fixes

[vector-of-bool]

This changeset comes from tentative inclusion in mongo-c-driver for the csfle patch, which uncovered a few quirks and possible improvements:

• mlib.cmake to define an import the library and its tests as a one-liner in the parent project.
• mlib_inline is a platform-specific inline keyword
• mlib_inline_def is a psuedo-keyword function attribute that mimics the link-once behavior of C++ inline (using weak linkage on ELF/macOS, while COFF (Windows) inline always has this behavior).
• MinGW is missing a flag definition for WideCharToMultiByte (discovered in the MinGW target of the C driver's build).