bash: optionally code-sign the executable
bash: optionally code-sign the executable

If the environment variable `SIGNTOOL` is defined, interpret it as the
command to use to code-sign these files.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

dscho/MSYS2-packages,git-for-windows/MSYS2-packages,git-for-windows/MSYS2-packages,dscho/MSYS2-packages,dscho/MSYS2-packages,dscho/MSYS2-packages,git-for-windows/MSYS2-packages,git-for-windows/MSYS2-packages