Define 'nncc_find_package' as macro (#149)

Define 'nncc_find_package' as macro (#149)

In CMake, function creates a new scope, and thus it interfers the
behavior of 'find_package' command if its wrapper ('nncc_find_package')
is implemented as a function.

This commit rewrites 'nncc_find_package' as a macro (which does not
create a new scope) to avoid scope issue, and updates existing
configurations accordingly.

Signed-off-by: Jonghyun Park <jh1302.park@samsung.com>