a few settings for compilation

1. some versions of ninja cannot display diagnostics when using clang
compilation, using `-fcolor-diagnostics' to force that
2. add additional warning flags when the compiler is
clang/gcc
3. add /build/ into .gitignore for out-of-source compilation(as is the
recommended approach to compile cmake projects)
minor fix for rtags-get-summary-text

rtags-display-summary sometimes gets errors when the "cursorinfo" is
nil(e.g., when the point is inside "#include <iostream>"); the change is
simply adding a check inside rtags-get-summary-text. However, the better
way for this case is to display the location of file `iostream'.

Another change is to replace the keymap of rtags-print-cursorinfo for
rtags-display-summary.
Add C-x r S as rtags-display-summary.