Select optimisation level per dependency

[kleisauke]

Use a default optimisation level of -Os, selectively compile with -O3 for dependencies that benefit.

Resolves: #55.

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Here's a size comparison for the Windows x64 binaries (statically-linked "web" variant).

Before (commit 41e8e1c1cdef5e6e1e50e69b62cd7c1378cff82a):

$ du -sbc *.dll
1184256     libglib-2.0-0.dll
243712      libgobject-2.0-0.dll
19117568    libvips-42.dll
456192      libvips-cpp-42.dll
21001728    total

After (this PR):

$ du -sbc *.dll
1012736     libglib-2.0-0.dll
215552      libgobject-2.0-0.dll
17845248    libvips-42.dll
456192      libvips-cpp-42.dll
19529728    total

So, a ~7% reduction in binary size.