The previous behavior generated a static library by default, and allowed
for building a shared library by setting -DBUILD_SHARED_LIBS=ON on the
cmake command line when building secp256k1 standalone.
It was not possible to build both a static and a shared library at the
same time, which would unfortunately prevent running the secp256k1 Java
tests as part of the check-all target, because these tests require a
shared secp256k1 library.
This diff is a first step toward building the JNI binding and running
the Java tests. It still defaults to building a static secp256k1 library
and add an option (SECP256K1_BUILD_SHARED) for building the shared