build: Fix support for PKG_CONFIG

- Allow the user to use PKG_CONFIG but not PKGCONFIG.

Background:

Last week in 14d5a86b a change was made to allow the user to set the
PKGCONFIG variable. Today in 72d99f2e I supplemented that to allow the
more common PKG_CONFIG as an alternative if PKGCONFIG is not set.

Neither of those changes worked as expected because PKGCONFIG is
occasionally reset in configure and by the CURL_CHECK_PKGCONFIG macro.
Instead in this commit I take the approach that the user may set
PKG_CONFIG only.