Link frameworks/libraries differently based on `:linkage` option

[trinhngocthuyen]

If not specified, the linkage is assumed to be :static.

• For :dynamic: Link against Foo.framework in ${PODS_CONFIGURATION_BUILD_DIR}/PackageFrameworks
• For :static: Link against Foo.o in ${PODS_CONFIGURATION_BUILD_DIR}

A package may have multiple products of multiple linkage types. For example, Foo is dynamic while Bar is static. In that case, we specify the :linkage option as follows:

spm_pkg "FooBar", 
        ...
        :linkage => { "Foo" => :dynamic, "Bar" => :static }