cannot build shared library in ncnn default cmake envs.

[kalcohol]

"target platform does not support dynamic linking” was shown when cross-build ncnn on linux for arm.
it seems like same issue on stackoverflow:
https://stackoverflow.com/questions/12264299/cmake-on-linux-target-platform-does-not-support-dynamic-linking/12266163

i add a line before order cmake build shared library, such as:
SET_PROPERTY(GLOBAL PROPERTY TARGET_SUPPORTS_SHARED_LIBS TRUE)

it was not a good solution, but just work well for now. please check and fix this issue when some one have time.
(i'm not good at cmake)

[nihui]

the ncnn ABI is not stable enough to be built as shared library.
so we provide static building only.

[Serbipunk]

the ncnn ABI is not stable enough to be built as shared library.
so we provide static building only.

unofficial old news of 01/01/2021:

ncnn shared library is supported!