pcre: Make it keg_only

Some packages (e.g., grep and wget, and who knows how many
others) that don't depend on pcre, and whose brew formulas don't
depend_on pcre, nevertheless silently pick up pcre during
configuration anyway.  This causes those packages to break if the
user---who has no idea that some installed kegs are linked to
pcre---later uninstalls the pcre keg.

I tested this out by doing `brew install $(brew uses pcre)` once
I changed pcre to keg_only.  Almost everything built and those
that didn't were not for pcre-related reasons.

Any other packages that really do need pcre should gain a
`depends_on 'pcre'`.