muon: undefine _FORTIFY_SOURCE

Fvwm3 needs work to compile with _FORTIFY_SOURCE -- disable for now.

meson.build

@@ -27,6 +27,16 @@ add_project_arguments(
 meson.add_dist_script('meson-scripts/dist-tarball.sh')
 
 cc = meson.get_compiler('c')
+# Undefine _FOTIFY_SOURCE explicitly.  fvwm3 needs work to support this.
+cc_flags = [
+    '-U_FORTIFY_SOURCE'
+]
+foreach flag: cc_flags
+    if cc.has_argument(flag)
+        add_project_arguments(flag, language: 'c')
+    endif
+endforeach
+
 host = host_machine.system() # This is the target system when cross compiling
 
 m_dep = cc.find_library('m', required: false)