gpgrt-config: Don't support variable reference in --libdir option.

* src/gpgrt-config: Simplify --libdir support, remove variable
substituion.
* src/gpg-error.m4: Expand possible ${prefix}, ${exec_prefix} in
libdir.  Simplify invocation of gpgrt-config.
* src/gpgrt.m4: Likewise.

The --libdir option for configure can include variable reference
like --libdir='${exec_prefix}/lib/x86_64-linux-gnu', and this
expression should be expanded (the variable should be substituted)
beforehand to determine if there is gpg-error.pc file or not.

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