You're supposed to do this:
gcc exa_7.c -o exa_7 -std=c99 `mysql_config --cflags --libs` `pkg-config --libs --cflags gtk+-2.0`
If the backticks are causing you problems, you can run the programs using $() instead:
gcc exa_7.c -o exa_7 -std=c99 $(mysql_config --cflags --libs) $(pkg-config --libs --cflags gtk+-2.0)