Build system tries to get ncnn's dependencies and link with them
likely(?) expecting a static ncnn. These checks have caused issues
before [1] and even if that was fixed, there is no reason to do
this and DEPEND for nothing.

Cannot do -DUSE_SYSTEM_NCNN=no given there is another condition
using that that we do need.

[1] https://bugs.gentoo.org/922512
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -53,3 +53,3 @@
 
-if(USE_SYSTEM_NCNN)
+if(0)
     find_package(Threads)