- limit the use of an asm optimisation for `std::sqrt()' to x86 archs,
since it breaks the build everywhere else.
- remove `-fopenmp' for base-gcc archs; it breaks the build on them
- remove `-Wno-c++11-narrowing' from CXXFLAGS; no occurrence of it has
been found, and such warning is notably useful for arm* and powerpc.
Tested on sparc64 (kmos@), macppc and amd64 (me).
OK kmos@
9d759c4e38