In v1.76.0 Boost.Regex became a header-only library.
With this update, there are now two different versions:
- v4 for C++03 (deprecated)
- v5 header-only

This commit fixes an issue which was preventing Boost.Regex
from being built for old ArmV5 targets

Signed-off-by: Carlos Miguel Ferreira <carlosmf.pt@gmail.com>