Add compile_commands.json file to .gitignore.

The file is used to describe project compilation independent
of build system in use.

It can also help LSP servers to improve code suggestions since
it can see how the project is configured and respect, for example,
content of config.h file.

.gitignore

@@ -17,6 +17,7 @@ ABOUT-NLS
 aclocal.m4
 autom4te.cache/
 compile
+compile_commands.json
 config.guess
 config.h
 config.h.in