qmmp, qmmp-plugin-pack: Fix staging path

Do not use 'USES= qmake' that overwrites the environmental
value INSTALL_ROOT with path same as STAGEDIR, in other
word, this USES= macro will not allow any other value to
be set on this value.

  - Revert do-configure: procedure that any other macro do
    not provide a configure command using qmake.
  - Set INSTALL_ROOT=${STAGEDIR}${PREFIX} on MAKE_ENV