GCC 7.1 (mingw-w64) has broken -flto and hypot

[RedSkotina]

For future:
GCC 7.1 (mingw-w64)
Just examine problem while compiling tests with unreferenced symbols on Release and MinSizeRel .
-flto have mystic conflicts with -O3 or -Os
Temp solution while gcc developers not fix bug:
Disable -flto flag like this:
cmake .. -G"Ninja" -DPYBIND11_LTO_CXX_FLAGS=""

mingw-w64 headers have conflict with python headers:
error: '::hypot' has not been declared
we can fix it with redefining -D_hypot=hypot
cmake .. -G"Ninja" -DPYBIND11_LTO_CXX_FLAGS="" -DCMAKE_CXX_FLAGS="-D_hypot=hypot"

[dean0x7d]

I assume this is informational since these are bugs related to mingw-w64 and not specifically pybind11. I'll close the issue for bookkeeping, but it will still show up in searches so others will be able to use the workarounds you suggested.