[bugfix] Make sure avx2 defines are set for compilation if supported.

Previously this was done via config.h, but due to the recent build
system refactoring these defines became MIA and some  code is never
activated even if it would be supported by the hardware.

We now set the necessary compile definitois as properties for the relevant
source files to fix this.

CMakeLists.txt

@@ -327,6 +327,7 @@ macro(opm-simulators_sources_hook)
 
   if(HAVE_AVX2_EXTENSION)
     set_property(SOURCE ${AVX2_SOURCE_FILES} PROPERTY COMPILE_OPTIONS ${AVX2_FLAGS})
+    target_compile_definitions(opmsimulators PUBLIC HAVE_AVX2_EXTENSION=1)
   endif()
 
   # Boost 1.66 used on RHEL8 cannot be built as c++-20