Wrap preprocessor definition of NOMINMAX in ifndef conditional to suppress warning when cross compiling Windows.
fs.cpp:6:0: warning: "NOMINMAX" redefined
/usr/lib/gcc/x86_64-w64-mingw32/7.3-posix/include/c++/x86_64-w64-mingw32/bits/os_defines.h:45:0: note: this is the location of the previous definition #define NOMINMAX 1
#define NOMINMAX was introduced in the following merge.