i486 build defaults to -march=i686 even if -march is specified via CXXFLAGS

[kwinsch]

I export the CFLAGS, CXXFLAGS and ASFLAGS via a conan profile, which includes some default CXXFLAGS, including the correct -march, -mtune and other optimizations for an intel Atom prozessor. Unfortunatly, the boost build system is defaulting to i686, even when -march=silvermont -mtune=silvermont -mfpmath=sse (Bay Trail Intel Atom) is specified. The reason seems to be in https://github.com/boostorg/build/blob/develop/src/tools/gcc.jam#L1190 . Unfortunatly the list is outdated and is not matching the current one, valid for gcc 8.3+.

I suggest to default to the supplied -march, if specified via the CXXFLAGS and only default to i686, if the information is missing.

[SSE4]

/cc @grafikrobot

[Lastique]

FYI, you can select the target instruction set by adding instruction-set=... to b2 command line.

[stale[bot]]

Thank you for your contributions. Main development of B2 has moved to https://github.com/bfgroup/b2 This issue has been automatically marked as "transition" to indicate the potential for needing transition to the new B2 development project.