Temporary header file '/tmp/I9ype5RQqB/dump1.h' with the following content will be compiled to create GCC translation unit dump:

  // add defines
  #define __STDC_VERSION__ 201710L
#define _GXX_NULLPTR_T
#define CLUTTER_ENABLE_COMPOSITOR_API 1
#define _REGEX_NELTS(n)
#define _Atomic
#define PACKED
#define getaddrinfo_a(a,b,c,d)

  // add includes
  #include "/usr/include/mgl2/Fl_MathGL.h"
  #include "/usr/include/mgl2/abstract.h"
  #include "/usr/include/mgl2/addon.h"
  #include "/usr/include/mgl2/base.h"
  #include "/usr/include/mgl2/base_cf.h"
  #include "/usr/include/mgl2/canvas.h"
  #include "/usr/include/mgl2/canvas_cf.h"
  #include "/usr/include/mgl2/canvas_wnd.h"
  #include "/usr/include/mgl2/config.h"
  #include "/usr/include/mgl2/cont.h"
  #include "/usr/include/mgl2/data.h"
  #include "/usr/include/mgl2/data_cf.h"
  #include "/usr/include/mgl2/datac.h"
  #include "/usr/include/mgl2/datac_cf.h"
  #include "/usr/include/mgl2/define.h"
  #include "/usr/include/mgl2/dllexport.h"
  #include "/usr/include/mgl2/eval.h"
  #include "/usr/include/mgl2/evalc.h"
  #include "/usr/include/mgl2/fit.h"
  #include "/usr/include/mgl2/fltk.h"
  #include "/usr/include/mgl2/font.h"
  #include "/usr/include/mgl2/glut.h"
  #include "/usr/include/mgl2/mgl.h"
  #include "/usr/include/mgl2/mgl_cf.h"
  #include "/usr/include/mgl2/mpi.h"
  #include "/usr/include/mgl2/opengl.h"
  #include "/usr/include/mgl2/other.h"
  #include "/usr/include/mgl2/parser.h"
  #include "/usr/include/mgl2/pde.h"
  #include "/usr/include/mgl2/plot.h"
  #include "/usr/include/mgl2/prim.h"
  #include "/usr/include/mgl2/qmathgl.h"
  #include "/usr/include/mgl2/qt.h"
  #include "/usr/include/mgl2/surf.h"
  #include "/usr/include/mgl2/thread.h"
  #include "/usr/include/mgl2/type.h"
  #include "/usr/include/mgl2/vect.h"
  #include "/usr/include/mgl2/volume.h"
  #include "/usr/include/mgl2/window.h"
  #include "/usr/include/mgl2/wnd.h"
  #include "/usr/include/mgl2/wnd_cf.h"
  #include "/usr/include/mgl2/wx.h"

The GCC parameters:
  gcc -fdump-lang-raw -fkeep-inline-functions -c -x c++ -fpermissive -w "/tmp/I9ype5RQqB/dump1.h"

In file included from /tmp/I9ype5RQqB/dump1.h:12:
/usr/include/mgl2/Fl_MathGL.h:27:10: fatal error: FL/Fl.H: No such file or directory
   27 | #include <FL/Fl.H>
      |          ^~~~~~~~~
compilation terminated.