[trunk] Convert CMake-language commands to lower case

[trunk] Convert CMake-language commands to lower case
Ancient CMake versions required upper-case commands.  Later command names became case-insensitive.  Now the preferred style is lower-case.
This also changes all the key words to lower case. The primary reason for changing key words is that all documentation for CMakeLists.txt now shows the key words as lower case. Even the printed “Mastering CMake v5” uses lower case.
Thanks to Hans Johnson