Tests show that EL6 is too old to support `%elif` statement (which is very stupid).
And revert dependency `openssl-libs` to `openssl` because of
inconsistent names in EL6.
The following error will occur when configuring with --enable-system-shared-lib:
```
/usr/bin/rm: cannot remove 'libtoolT': No such file or directory
```
This issue is caused by overwriting gnu make variable `RM` with `rm`,
whose default value is `rm -f` (https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html).
Rayson zhu