bootstrap: Use 'tr' more portably

Since commit 380bd70cc2 (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.

bootstrap

@@ -38,7 +38,7 @@ cmake_install_dest_default()
 
 cmake_toupper()
 {
-    echo "$1" | tr '[:lower:]' '[:upper:]'
+    echo "$1" | tr '[a-z]' '[A-Z]'
 }
 
 # Detect system and directory information.