[libc++][PowerPC] Fix a test case failure when compiled with libcxx

The test case is not ran unless libcxx is used, and a macro
may be undefined. This patch checks for the definition of the
macro before using it.

Fixes http://llvm.org/PR51430

Differential Revision: https://reviews.llvm.org/D108352

libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.float.pass.cpp

@@ -92,7 +92,7 @@ void fp_test()
     assert(d0 < d1);  // sanity checking
     assert(d1 < d2);  // sanity checking
 
-#if defined(__PPC__) && __LONG_DOUBLE_128__ && !__LONG_DOUBLE_IEEE128__
+#if defined(__PPC__) && __LONG_DOUBLE_128__ && !(defined(__LONG_DOUBLE_IEEE128__) && __LONG_DOUBLE_IEEE128__)
 //	For 128 bit long double implemented as 2 doubles on PowerPC,
 //	nextafterl() of libm gives imprecise results which fails the
 //	midpoint() tests below. So skip the test for this case.