bootstrap: Use 'tr' more portably

Since commit 380bd70c (bootstrap: implement cmake_toupper() using tr,
2017-06-07, v3.10.0-rc1~548^2~2) we use `tr` to convert from lower to
upper case.  However, the character classes `[:lower:]` and `[:upper:]`
result in a "Bad string" error message on Solaris.  Use `[a-z]` and
`[A-Z]` instead.