Reduce number of exported symbols in shared library

[mreineck]

At one of the last meetings @ahbarnett noted that the number of symbols exported from libfinufft.so increases by a lot when ducc is used for FFTs.
This can most likely be fixed by compiling the sources with "-fvisibility=hidden" under Linux and using the already existing FINUFFT_EXPORT macros to explicitly make functions visible under Linux, very similar to what happens under Windows.

The current state of the branch demonstrates this when compiling with the makefile; you can check the resulting shared library using nm -C -D -U lib/libfinufft.so to see the exported symbols.
I don't think there is much I can do about the exported weak symbols (w/W), but the overall number goes down from 1000+ to about 130 when using the ducc back end, which is at least some progress.

Still to do:

• support this in cmake
• see whether there are non-Windows compilers which don't understand the visiblity flags/attributes, and disable the mechanism for them.