[MatL] Set USE_MKL according to cmakes OGS_USE_MKL

Fixes #3075

MathLib/CMakeLists.txt

@@ -61,5 +61,5 @@ target_compile_definitions(MathLib
     $<$<BOOL:${OGS_USE_EIGEN}>:OGS_USE_EIGEN>
     $<$<BOOL:${CVODE_FOUND}>:CVODE_FOUND>
     PRIVATE
-    $<$<BOOL:${USE_MKL}>:USE_MKL>
+    $<$<BOOL:${OGS_USE_MKL}>:USE_MKL>
 )