igraph: enable autodetection for LTO support (#122058)

> The IGRAPH_ENABLE_LTO build option now supports the AUTO value, which
> uses LTO only if the compiler supports it. Warning: CMake may not
> always be able to detect that LTO is not fully supported. Therefore,
> the default setting is OFF.

https://igraph.org/2021/03/23/igraph-0.9.1-c.html

pkgs/development/libraries/igraph/default.nix

@@ -77,15 +77,17 @@ stdenv.mkDerivation rec {
     "-DIGRAPH_USE_INTERNAL_GMP=OFF"
     "-DIGRAPH_GLPK_SUPPORT=ON"
     "-DIGRAPH_GRAPHML_SUPPORT=ON"
-    "-DIGRAPH_ENABLE_LTO=ON"
+    "-DIGRAPH_ENABLE_LTO=AUTO"
     "-DIGRAPH_ENABLE_TLS=ON"
     "-DBUILD_SHARED_LIBS=ON"
   ];
 
   doCheck = true;
 
-  preCheck = ''
-    # needed to find libigraph.so
+  # needed to find libigraph, and liblas on darwin
+  preCheck = if stdenv.isDarwin then ''
+    export DYLD_LIBRARY_PATH="${lib.makeLibraryPath [ blas ]}:$PWD/src"
+  '' else ''
     export LD_LIBRARY_PATH="$PWD/src"
   '';