minor, check for user CFLAGS
chore, for user-specified -O or -march or -mtune values, don't check for those flags in SXE_OPTIFLAGS
minor, don't set -march/-mtune/-xHost/-fast upon user-specified CFLAGS
fix, pair -xHost with -march/-mtune checking
fix, PEBKAC, actually use user specified CFLAGS
Merge branch 'bld/configure'

* bld/configure:
  fix, PEBKAC, actually use user specified CFLAGS
  fix, pair -xHost with -march/-mtune checking
  minor, don't set -march/-mtune/-xHost/-fast upon user-specified CFLAGS
  chore, for user-specified -O or -march or -mtune values, don't check for those flags in SXE_OPTIFLAGS
  minor, check for user CFLAGS
chore, adapt coru macros so old compilers can cope
Merge branch 'bld/old-gcc'

* bld/old-gcc:
  chore, adapt coru macros so old compilers can cope