meson when compiling something with 'native: true' (for building
executables that are meant to be run on the host system) uses CFLAGS
and CXXFLAGS environment variables, we "pollute" them with
TARGET system cflags/cxxflags, such as -march= for the cross-compiled
arch.
so to fix it we export CFLAGS and CXXFLAGS to be CFLAGS_host and
CXXFLAGS_host respectively, they are set by XBPS and correspond to
the XBPS_CFLAGS/XBPS_CXXFLAGS. This same set of changes is also done
with CC and CXX see L#61
this was found when cross-compiling lighttpd which created the
'lemon' executable to generate inputs
thanks to @Cogitri from Exherbo for helping me debug this
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