Fix compat for julia < v1.5

Author: inkydragon

src/manual_wrappers.jl

@@ -35,7 +35,11 @@ gsl_function_helper(x::Cdouble, fn)::Cdouble = fn(x)
 # The following code relies on `gsl_function` being a mutable type
 # (such that we can call `pointer_from_objref` on it) to simplify the object structure
 # a little bit and avoid hitting some limitation of the allocation optimizer.
-@assert ismutable(gsl_function(C_NULL, C_NULL))
+@static if VERSION >= v"1.5"
+    @assert ismutable(gsl_function(C_NULL, C_NULL))
+else
+    @assert !isimmutable(gsl_function(C_NULL, C_NULL))
+end
 
 function wrap_gsl_function(fn::F) where F
     # We need to allocate the `gsl_function` here to be kept alive by ccall