build: allow disabling the GTK interface with --disable-gtk

For some systems, such as embedded Linux including ARM, we might want to
just compile libsysprof/sysprof-cli without the GTK user interface. This
allows for that. You can copy the capture files to your visualization
host to render the results.

src/Makefile.am

@@ -1,3 +1,5 @@
+if ENABLE_GTK
+
 bin_PROGRAMS = sysprof
 
 sysprof_CFLAGS = \
@@ -37,4 +39,6 @@ glib_resources_h = sp-resources.h
 glib_resources_namespace = sysprof
 include $(top_srcdir)/gresources.mk
 
+endif
+
 -include $(top_srcdir)/git.mk