doc,configure: Be consistent about preferring --with-libgpg-error-prefix

* doc/gpgrt.texi: Say "--with-libgpg-error-prefix" instead of
  "--with-gpg-error-prefix".
* src/gpg-error.m4: When warning about library locations, warn with
  the preferred "--with-libgpg-error-prefix" name.

in src/gpg-error.m4, it already says:

dnl --with-libgpg-error-prefix=PFX is the preferred name for this option,
dnl since that is consistent with how our three siblings use the directory/
dnl package name in --with-$dir_name-prefix=PFX.

so this is an attempt to live up to that expectation.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>