common: Update requisites

* configure.ac: Require libgpg-error 1.38 and libksba 1.4.0
* common/util.h: Remove error number substitutes.

These versions were already needed for a successful compilation,
but it was not reflected by configure.

util.h patch by wk.

• GnuPG-bug-id: T6974
• Signed-off-by: Ángel González <angel@pgp.16bits.net>