- Updated configure script to be a lot faster when searching for ANTL…

…R headers (look in ANTLRHOME/include and ANTLRHOME/lib instead of searching all of /usr).

- Fixed an issue where ANTLRHOME can be set to /home/user/src/ but the script finds the headers in /usr/local and Makefiles then fail.
- You may now have ANTLRHOME unset when doing ./configure.
  - The default ANTLRHOME is /usr.
- Readme file updated to reflect these changes.


git-svn-id: https://openmodelica.org/svn/OpenModelica/trunk@5277 f25d12d1-65f4-0310-ae8a-bbce733d8d8e