automake: do not use symbols names for static glapi.la

automake: do not use symbols names for static glapi.la

In the cases where one links against the static glapi.la there
is no need to create temporary variables only to explicitly
link agaist it.

Instead use SHARED_GLAPI_LIB to explicitly indicate when one
is building and linking with the shared glapi provider.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>