cmake: use BUILD_SHARED_LIBS instead of ENABLE_STATIC to drive the shared object build

In case ENABLE_STATIC is still set on the CMake command line, this
change will makes CMake bail out becaus e this option is no longer
supported.

This patch makes taglib more compliant with the distro package framework
that uses the standards CMake options.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>