[PATCH] simplify case insensitive file system detection logic in configure

[Qian Yun]

I added the case insensitive file system detection logic in
configure, later a --enable-case-insensitive-file-system-check
option is added due to sagemath's requirement.

Turns out this can be simplified:

The only problematic situation is:
1. in-tree build
2. and build without pre-generated (aka non release tarball build)
3. and on a case insensitive file system

So I removed the --enable-case-insensitive-file-system-check
option and only do the check in configure when pre-generated
is not available.

- Qian

[Waldek Hebisch]

OK, please commit.
--
Waldek Hebisch