Use system compiler flags if set

[mikhailnov]

When building an RPM package system compiler flags are set. They include -g/-gdwarf-4 to enable debuginfo which is futher stripped automatically and put into a separate package into a separate repository (https://mirror.rosalinux.ru/rosa/rosa2021.1/repository/x86_64/debug_contrib/release/)

Append system flags instead of ignoring them.

[mikhailnov]

P.S. I wanted to package https://github.com/Haivision/srt but thought that this repository is what I need :xD