[fftw3]Changed library linkage to static.

[JackBoosY]

Because many functions are not declared as exportable in fftw3, we can only build them as static libraries now.
The following are these functions(prefix with X in the declaration):

• imax
• mksolver_ct_hook
• mksolver_hc2hc_hook
• mksolver
• mkplan_d
• mkplan_dft
• mkplan_rdft
• mkplan_rdft2
• mktensor_1d
• mktensor_2d
• mkproblem_dft
• mkproblem_dft_d
• mkproblem_rdft
• mkproblem_rdft_d
• mkproblem_rdft2
• solvtab_exec
• dft_solve
• ops_zero
• ops_add2
• tensor_tornk1
• plan_destroy_internal
• choose_radix
• ct_applicable
• tensor_copy
• tensor_destroy
• solver_register
• pickdim
• rdft_solve
• hc2hc_applicable
• rdft2_solve
• rdft2_inplace_strides
• rdft2_strides
• set_planner_hooks

Relate: #6056.

[alessandroarduino]

After this merge, I can no more link my C++ code to FFTW3 with msvc compilers (both using Visual Studio projects and CMake projects, in Windows 10). Errors like

error LNK2019: unresolved external symbol __imp_fftw_execute referenced in function [...]

arise.

I have to restore the previous version of file ports/fftw3/portfile.cmake in order to be able to produce my executables.