CMakeDetermineCompilerABI: Avoid removing the flag after -Werror

The matching became too eager after commit 079ea66468
(CMakeDetermineCompilerABI: Handle NVCC-style -Werror flags, 2020-10-04,
v3.19.0-rc1~45^2). 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: 079ea66468a6ffe0b02c3d6622bc0230fdf455b0
See-also: https://discourse.cmake.org/t/8230