Merge pull request #9 from ezequielv/fix-install_to_nonexisting_prefix-01

fixed: Makefile.in: specifying a nonexisting directory to '--prefix' (in 'configure') now works
Added elvgdb to .gitignore.
Added XQuartz as default for darwin builds with X11.
Fixed what I think is a bug: empty statement after `if'.
Fixed compiler warning.
Fixed broken error detection for opening files on Unix.
Merge pull request #7 from ezequielv/fix-configure_x11_libs_redhat_6.6-01

configure: fixed X11 libraries detection on RedHat/CentOS
Fixed compiler warning.
configure script:
- added support for different shared library suffixes
- better handling of darwin/OSX particularities
Merge branch 'master' of https://github.com/mbert/elvis