Enable static targets

[IsabelParedes]

When building static libraries only (e.g. emscripten-forge), find_package(xvega REQUIRED) fails because there are no static targets installed.

CMake Error at $PREFIX/lib/cmake/xvega/xvegaConfig.cmake:45 (include):
  include could not find requested file:
    $PREFIX/lib/cmake/xvega/xvegaTargets.cmake