gpgrt.m4: Don't support --with-*-prefix or SYSROOT.

* src/gpgrt.m4: Remove old backward compatible support.

Since this macro gpgrt.m4 has not yet been used by any, we can
break now. The gpgrt-config script is for all hosts (switching
by --libdir option), it's not switched by --with-*-prefix nor
SYSROOT.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>