libgfortran bootstrap failure

[David Edelsohn]

The recent patch to support Power IEEE128 causes a bootstrap failure
on AIX and possibly all non-GLIBC systems.

+#if defined(__powerpc64__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ \
+    && defined __GLIBC_PREREQ && __GLIBC_PREREQ (2, 32)
+#define POWER_IEEE128 1
+#endif

__GLIBC_PREREQ is tested on all systems.

/nasfarm/edelsohn/src/src/libgfortran/libgfortran.h:107:49: error:
missing binary operator before token "("
  107 |     && defined __GLIBC_PREREQ && __GLIBC_PREREQ (2, 32)
      |                                                 ^