Change CMAKE_BUILD_TYPE comparison to be case-insensitive (#2130)

Change CMAKE_BUILD_TYPE comparison to be case-insensitive (#2130)

Currently the `CMAKE_BUILD_TYPE` is being compared in a case-sensitive
way. It seems current CMake documentation [suggests treating this in a
case-insensitive manner
now-a-days](https://cmake.org/cmake/help/latest/manual/cmake-buildsystem.7.html#case-sensitivity).

This being case-sensitive creates needless complexities in other
projects if they compile their own project with `cmake
-DCMAKE_BUILD_TYPE=release ..`, etc. In this specific case, libarchive
has a fatal error due to the lowercase `release`.

I'd honestly like to remove these comparisons entirely; as I'm not sure
if they're really needed or not if `libarchive` is only using the
Makefile or Ninja generators with CMake.

This PR changes the `CMAKE_BUILD_TYPE` comparison to be
case-insensitive, and leaves the rest alone.

This should also fix the following issue(s):
* https://github.com/libarchive/libarchive/issues/1792