Allow specifying dependencies from subprojects

[lpereira]

This patch makes it possible for users of the add_builtins() CMake function to specify a per-builtin dependency on another target. This has been used to fix #42.

[guybedford]

I can confirm this resolves the issue for us.

[lpereira]

Force-pushed changes that:

1. Remove LINK_LIBS
2. Establish DEPENDENCIES as a more general-purpose way of declaring the dependencies of built-ins
3. Installs OpenSSL in ${CMAKE_BINARY_DIR}/deps/OpenSSL, so it's clearer that it's a dependency
4. Patches usages of LINK_LIBS in builtins.cmake to use DEPENDENCIES instead
5. Uses a VERBOSE message when printing information about a builtin's dependencies

[guybedford]

Again I can confirm that the latest updates all work correctly in the Fastly branch.

[lpereira]

Rebased and force-pushed with the clarifications.