Problem with Debian 10, Eclipse, C when linking GTK3 libs - eclipse

Question

I am using Debian 10, GTK3 (package libgtk-3-dev), CDT, GCC. Compiling and linking by command line / shell ($1-parameter) works fine:
gcc `pkg-config --cflags gtk+-3.0` $1.c -o $1 `pkg-config --libs gtk+-3.0`
I installed Codeblocks and changed the compiler and linker settings due to command line options, it works as well.
But I want to use Eclipse (2019-03) as IDE as I am used to it.
Installation of pkg-config plugin via Marketplace failed (compatibilty problem).
So I changed compiler and linker settings, replacing pkg-config (didn't work) with its contents.
Compiler settings:
-pthread -I/usr/include/gtk-3.0 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/at-spi-2.0 -I/usr/include/dbus-1.0 -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include -I/usr/include/gtk-3.0 -I/usr/include/gio-unix-2.0 -I/usr/include/cairo -I/usr/include/libdrm -I/usr/include/pango-1.0 -I/usr/include/harfbuzz -I/usr/include/pango-1.0 -I/usr/include/fribidi -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/uuid -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -lgtk-3 -lgdk-3 -lpangocairo-1.0 -lpango-1.0 -latk-1.0 -lcairo-gobject -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0
Linker settings:
-lgtk-3 -lgdk-3 -lpangocairo-1.0 -lpango-1.0 -latk-1.0 -lcairo-gobject -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0
Program:
#include <gtk/gtk.h>
void on_button_clicked (GtkButton *button) {
g_print ("button '%s' clicked\n", gtk_button_get_label (button));
}
int main (int argc, char *argv[]) {
GtkWidget *window, *button;
gtk_init (&argc, &argv);
window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
gtk_container_set_border_width (GTK_CONTAINER (window), 10);
button = gtk_button_new_with_label ("Hallo Michel");
gtk_container_add (GTK_CONTAINER (window), button);
g_signal_connect (window, "destroy", G_CALLBACK (gtk_main_quit), NULL);
g_signal_connect (button, "clicked", G_CALLBACK (on_button_clicked), NULL);
gtk_widget_show_all (window);
gtk_main ();
return 0;
}
Compilation works (.o-file created), debugging is possible, but linking fails:
Eclipse Console output:
Building target: GTK3
Invoking: GCC C Linker
gcc -lgtk-3 -lgdk-3 -lpangocairo-1.0 -lpango-1.0 -latk-1.0 -lcairo-gobject -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0 -o "GTK3" ./gtk_test.o
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/Scrt1.o: in function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
make: *** [makefile:31: GTK3] Fehler 1
"make all" terminated with exit code 2. Build might be incomplete.
Eclipse Problems output:
/usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/Scrt1.o: in function `_start': GTK3 C/C++ Problem
make: *** [makefile:31: GTK3] Fehler 1 GTK3 C/C++ Problem
Type 'GTK_CONTAINER (window)' could not be resolved gtk_test.c /GTK3 line 14 Semantic Error
Type 'GTK_CONTAINER (window)' could not be resolved gtk_test.c /GTK3 line 17 Semantic Error
Type 'GTK_WINDOW (window)' could not be resolved gtk_test.c /GTK3 line 13 Semantic Error
undefined reference to `main' GTK3 C/C++ Problem
Who knows what to do?
Thanks in advance
Michel