use isdebugbuild() rather than ccalling into libjulia directly (#903)

Author: sjkelly

src/juliaconfig.jl

@@ -15,12 +15,12 @@ end
 
 function julia_private_libdir()
     if Base.DARWIN_FRAMEWORK # taken from Libdl tests
-        if ccall(:jl_is_debugbuild, Cint, ()) != 0
+        if isdebugbuild() != 0
             dirname(abspath(Libdl.dlpath(Base.DARWIN_FRAMEWORK_NAME * "_debug")))
         else
             joinpath(dirname(abspath(Libdl.dlpath(Base.DARWIN_FRAMEWORK_NAME))),"Frameworks")
         end
-    elseif ccall(:jl_is_debugbuild, Cint, ()) != 0
+    elseif isdebugbuild() != 0
         dirname(abspath(Libdl.dlpath("libjulia-internal-debug")))
     else
         dirname(abspath(Libdl.dlpath("libjulia-internal")))