Add support for building static executables

[CosmicToast]

This fixes the following issues:

• linked objects do not use LDFLAGS
• pkg-config is not queried appropriately depending on the desired linkage
• there's no way to configure/deconfigure static linking (except by setting LDFLAGS, which won't work due to the prior two)

There are many ways of fixing these, but I went for the minimally intrusive approach:

• any linked objects have $(LDFLAGS) in front of $(LIBS)
• configure has a --static and --dynamic flag
• --static is added to ldflags only if selected

If any changes are desired, please feel free to make them - what matters to me is the utility, moreso than the exact implementation details (so long as they accomplish the same things). This patch is pretty small, so if licensing is a concern, please consider it CC0.

[craigbarnes]

The sourcehut repo seems to be lacking this commit:

• https://git.sr.ht/~emersion/mrsh/log
• https://github.com/emersion/mrsh/commits/master