Compilation should target native platform

[tkittel]

Investigating McStas performance in conda envs with @willend, we concluded what in hindsight appears obvious: applications (like mcstas and simplebuild both) where users both compile+run code themselves, can benefit greatly from having the compilation targeting the native platform. In particular it might be useful to tack on -march=native -mtune=native. Investigating in a conda-forge env on ubuntu 22, we found that tacking the following flags all helped (the two first was for an executable obviously):

-fno-PIC -fPIE -flto -O3 -mtune=native -march=native -fno-math-errno -ftree-vectorize -g -DNDEBUG

Another related revelation we made is that while conda-forge provide a modern gcc-12 compiler, the std c library is in an ancient version. There is not much to do about that, but for C code using the defines in math.h, we saw extremely slow isnan and isinf due to some obnoxious macros, so it helped a lot to inject the following after math.h was included:

#if defined(__GNUC__) || defined(__clang__)
#  ifdef isnan
#    undef isnan
#  endif
#  ifdef isinf
#    undef isinf
#  endif
#  define isnan(x) __builtin_isnan(x)
#  define isinf(x) __builtin_isinf(x)
#endif

C++ code might not be affected by this.