contrib: Fix non-portable shell commands in gcc-git-customization.sh [PR102664]

Use printf instead of echo -n. Use Basic Regular Expressions instead of
sed -r. Check for error from ancient Git versions that don't support the
--git-path option for git-rev-parse. Remove -c flag from install
command, as it's ignored by GNU and BSD install, but means something
different for Solaris and AIX.

contrib/ChangeLog:

	PR other/102664
	* gcc-git-customization.sh: Fix non-portable commands.