Summary:
This is a squashed commit.

- Include cmath instead of math.h since the code references cmath
    functions with the std namespace specifier.
- ext_poppler is missing two defines in the cmake config which can cause
    compilation error with some mingw versions, so we patch it.
- Try to detect ext_boost dll name for mingw instead of hard-coding a specific version
- Use 32-bit fftw DLLs when compiling for x86
- Skip files ending in .debug when loading plugins. This is to support
    separating debug info from the executable files and not polluting the gdb
    output with tons of "xxx.debug has no json" messages.

Reviewers: rempt

Subscribers: woltherav

Tags: #krita

Differential Revision: https://phabricator.kde.org/D2856