* configure: Only build libopcodes shared if --enable-shared's value

    * configure: Only build libopcodes shared if --enable-shared's value
        was `yes', or was set to `*opcodes*'.
        * aclocal.m4: Likewise.

so `--enable-shared=libstdc++' doesn't result in building a shared libopcodes

only an interim fix, this is the incorrect formal approach since alocal.m4
itself is generated via what's in devo/libtool/libtool.m4; we need to find
a way to have that somehow hold the real fix.