Rebuild fish.pc as appropriate

Re-running ./configure will cause fish.pc to rebuild, in case any of the
paths changed. It looks like this actually won't rebuild the rest of
fish, but figuring out how to handle that is out of scope for this
commit.

More importantly, this will rebuild fish.pc when the version string
changes.
web_config: fix output if no abbreviations defined
Fix typo in abbr documentation

[skip ci]
env.cpp: correctly check variable_can_be_array

Closes #1746.