[SPARK-2358][MLLIB] Add an option to include native BLAS/LAPACK loader in the build

It would be easy for users to include the netlib-java jniloader in the spark jar, which is LGPL-licensed. We can follow the same approach as ganglia support in Spark, which could be enabled by turning on "SPARK_GANGLIA_LGPL" at build time. We can use "SPARK_NETLIB_LGPL" flag for this.

@pwendell Could you ping me after you finish updating the build file and help review this after? Thanks!