Fix C++ standards library portability issue.

[tresf]

Fixes zyn instrument playback on Mac for certain presets.

This PR is a partial cherry-pick of upstream 2.5 patches: zynaddsubfx/zynaddsubfx@417d49b, zynaddsubfx/zynaddsubfx@edca8ab

Quoting the author, @hselasky from 2014:

The std::polar template has no clear definition for the range of
the input parameters, and some C++ standard library implementations
don't accept negative amplitude among others. Define our own
FFTpolar template, which works like we expect it to.

Closes #4152

[Wallacoloo]

@tresf windows builds failed:

In file included from /home/travis/build/LMMS/lmms/plugins/zynaddsubfx/zynaddsubfx/src/Params/PADnoteParameters.h:28:0,
                 from /home/travis/build/LMMS/lmms/plugins/zynaddsubfx/zynaddsubfx/src/Params/PADnoteParameters.cpp:24:
/home/travis/build/LMMS/lmms/plugins/zynaddsubfx/zynaddsubfx/src/Params/../DSP/FFTwrapper.h: In instantiation of 'std::complex<_Tp> FFTpolar(const _Tp&, const _Tp&) [with _Tp = float]':
/home/travis/build/LMMS/lmms/plugins/zynaddsubfx/zynaddsubfx/src/Params/PADnoteParameters.cpp:618:94:   required from here
/home/travis/build/LMMS/lmms/plugins/zynaddsubfx/zynaddsubfx/src/Params/../DSP/FFTwrapper.h:64:22: error: 'isnan' was not declared in this scope
         if (isnan(__x))
                      ^
/home/travis/build/LMMS/lmms/plugins/zynaddsubfx/zynaddsubfx/src/Params/../DSP/FFTwrapper.h:64:22: note: suggested alternative:
In file included from /home/travis/build/LMMS/lmms/plugins/zynaddsubfx/zynaddsubfx/src/Params/../DSP/FFTwrapper.h:27:0,
                 from /home/travis/build/LMMS/lmms/plugins/zynaddsubfx/zynaddsubfx/src/Params/PADnoteParameters.h:28,
                 from /home/travis/build/LMMS/lmms/plugins/zynaddsubfx/zynaddsubfx/src/Params/PADnoteParameters.cpp:24:
/opt/mingw32/i686-w64-mingw32/include/c++/4.9.4/cmath:632:5: note:   'std::isnan'
     isnan(_Tp __x)
     ^

I'm not sure why the builds for other platforms are succeeding. Unless fftw3.h is providing its own isnan function at the global scope.

You can see that at some point after this commit, ZASFx changed these from isnan to std::isnan (https://github.com/zynaddsubfx/zynaddsubfx/blob/master/src/DSP/FFTwrapper.h) - may be worth trying to track down which commit did that and cherry-picking it as well.

[Wallacoloo]

Oh, wow. Github offers git blame functionality right on the website. This is the commit you want: zynaddsubfx/zynaddsubfx@edca8ab

Although, it's still using just sin and cos instead of std::sin and std::cos, but mingw didn't seem to be complaining about those for some reason.

[PhysSong]

For master, it would be enough to cherry-pick two commits into https://github.com/LMMS/zynaddsubfx and bump the submodule.

[tresf]

You can try. (Or I will). One of the files doesn't exist, not sure if that'll cause a conflict.

[tresf]

Yeah, it threw it into conflict... Resolved, pushed to lmms/zynaddsubfx. Documented here: https://github.com/LMMS/zynaddsubfx/wiki.