configure: enable the gallium loader only when needed

configure: enable the gallium loader only when needed

With the gallium megadrivers we've converted most ST to optionally
use either statically linked in or shared pipe-drivers.

The hardcoded switch forgot to conditionally enable the build of the
shared pipe-drivers which resulted in them being constantly build.

Cc: "10.3" <mesa-stable@lists.freedesktop.org>
Cc: James Ausmus <james.ausmus@intel.com>
Reported-by: James Ausmus <james.ausmus@intel.com>
Tested-by: James Ausmus <james.ausmus@intel.com>
Bugzilla: https://code.google.com/p/chromium/issues/detail?id=412089
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>