Fix a build issue due to the use of non-portable variables

Fix a build issue due to the use of non-portable variables

$< isn't supported in BSD make say. $(RM) is pre-defined in GNU make
though, not in BSD make say. so changed to check on configure if it's
pre-defined by make, otherwise set the appropriate command to $(RM).

This would be a workaround until it has the certain pre-defined value.