Are there any compiler tricks that will enable the building of a perl (on an otherwise non-accommodating machine) that provides such discernment ?

I don’t know; but for the benefit of newbies reading this thread, I just want to point out that this level of discernment (and more) is already available out-of-the-box on any (non-ancient) perl via the bignum pragma:

13:07 >perl -wE "say 'ok' if 1.0 + 5e-200 != 1.0;"

13:07 >perl -Mbignum -wE "say 'ok' if 1.0 + 5e-200 != 1.0;"
ok

13:07 >perl -wE "say 'ok' if 1.1 + 5e-200 != 1.1;"

13:07 >perl -Mbignum -wE "say 'ok' if 1.1 + 5e-200 != 1.1;"
ok

13:07 >perl -v

This is perl, v5.10.1 (*) built for MSWin32-x86-multi-thread
...

13:07 >
[download]

But no doubt the -Duselongdouble version is more efficient.