@Beesdeckar I too use custom makefile projects a lot, and #ifdefs has been very annoying. If you can build from source, I just tried following workaround to view compiler settings for custom makefile projects, and it worked for me.
diff --git a/src/plugins/compilergcc/compileroptionsdlg.cpp b/src/plugins/compilergcc/compileroptionsdlg.cpp
index 069d72649..947ea16f5 100644
--- a/src/plugins/compilergcc/compileroptionsdlg.cpp
+++ b/src/plugins/compilergcc/compileroptionsdlg.cpp
@@ -433,8 +433,6 @@ CompilerOptionsDlg::CompilerOptionsDlg(wxWindow* parent, CompilerGCC* compiler,
     {
         nb->RemovePage(2); // remove "Search directories" page
         nb->RemovePage(1); // remove "Linker settings" page
-        nb->RemovePage(0); // remove "Compiler settings" page
-        XRCCTRL(*this, "tabCompiler", wxPanel)->Show(false);
         XRCCTRL(*this, "tabLinker", wxPanel)->Show(false);
         XRCCTRL(*this, "tabDirs", wxPanel)->Show(false);
     }
With this, Compiler settings are enabled in Project -> Build options..
Add defines in #defines tab
Save  project, close and open it again, and code under the #define looks normal.