Disable LTO in CI build with old gcc

CMakeLists.txt

@@ -36,10 +36,16 @@ include(CheckIPOSupported)
 check_ipo_supported(RESULT ipo_available OUTPUT error LANGUAGES C)
 
 # Enable IPO by default, if available
+if(NOT DEFINED CMAKE_INTERPROCEDURAL_OPTIMIZATION)
   if(ipo_available)
     set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
   else()
-  message(WARNING "IPO is not supported, expect performance penalty")
+    message(WARNING "IPO is not supported, disabling")
+  endif()
+endif()
+
+if(NOT CMAKE_INTERPROCEDURAL_OPTIMIZATION)
+  message(WARNING "IPO is not enabled, expect performance penalty")
 endif()
 
 # Extra build type for AddressSanitizer (Asan)

nix/rt.nix

@@ -92,6 +92,9 @@ let
       stdenv = stdenv;
     }).overrideAttrs (old: {
       pname = old.name + "@" + stdenv.cc.cc.version;
+      cmakeFlags = old.cmakeFlags ++ [
+        "-DCMAKE_INTERPROCEDURAL_OPTIMIZATION=OFF"
+      ];
     });
 
     oldCompilers = [