Expose all math *l functions as aliases for the * functions when long double == double

[Steve Kargl]

On Thu, Dec 27, 2018 at 04:09:37PM -0600, Justin Hibbits wrote:
> When building editors/calligra on powerpc64 it failed with undefined
> references to erfl and erfcl.  Some digging it turns out that these
> functions aren't being exposed as weak symbols in libm on platforms
> where long double is 64 bits, because LDBL_MANT_DIG is not defined in
> the files defining these functions.  The attached patch fixes that
> case.  It's only been compile-tested for powerpc64, but should have no
> negative impact on any other platform.
> 
> - Justin

Looks OK to me.

-- 
Steve