Allow multiple values for configure ENABLE and WITH options helpers

${opt}_CONFIGURE_ENABLE and ${opt}_CONFIGURE_WITH now accept multiple arguments
and will be expanded accordingly, e.g.:

OPT_CONFIGURE_ENABLE= feature1 feature2 -- will add to CONFIGURE_ARGS:
OPT enabled: --enable-feature1 --enable-feature2
OPT disabled: --disable-feature1 --disable-feature2

Approved by: portmgr (bapt)