Use RUDIX_BUILD_STATIC=yes to build static package

Pass option RUDIX_BUILD_STATIC=yes to the make command to build a static package.
Remove references for ONLY_STATIC_LIBS.

Example:

cd Ports/libevent
make RUDIX_BUILD_STATIC=yes pkg test

Signed-off-by: Rudá Moura <ruda.moura@gmail.com>