CMakeDetermineCompilerABI: avoid removing the flag after -Werror
The matching became too eager after !5318 (merged).
When -Werror was specified without a value we would eat the following flag.
Prevent this by disallowing "-" as the first character of the flag's value.
Fixes: !5318 (merged)
See-also: https://discourse.cmake.org/t/8230
Backport: release
Edited by Brad King