Make sure `gdkconfig.h' exists after the rule is fired.
* gdk/Makefile.am (gdkconfig.h): Make sure `gdkconfig.h' exists after the rule is fired. (install-exec-local): Install gdkconfig.h only if the contents are different from the currently installed gdkconfig.h. Rationale: `make install-exec' is meant to install files that go into $exec_prefix, which includes `gdkconfig.h'. However, developers use `make install-exec' when they know that none of the headers have changed, to prevent needless recompiles of other packages. My previous change of installing `*config.h' during `make install-exec' undermines this, even though it is the right thing to do. So, in the spirit of "do the right thing first, optimize later", here's the optimization.