I was compiling fvwm and I got this error:
X11 libraries or header files could not be found.  Please make
sure the X11 development package is installed on your system.
If it is definitely installed, try setting the include and library
paths with the --x-include and --x-libraries options of configure.
Fvwm can not be compiled without the X11 development environment

Where can I find these files?