libgfortran: Correct FP feature macro checks

libgfortran: Correct FP feature macro checks

The *_HAS_* floating-point feature macros are defined as 0/1 rather than
#undef/#define settings by gcc/c-family/c-cppbuiltin.c.  Consequently we
choose to use infinity and NaN features even with non-IEEE-754 targets
such as `vax-netbsdelf' that lack them, causing build warnings and
failures like:

In file included from .../libgfortran/generated/maxval_r4.c:26:
.../libgfortran/generated/maxval_r4.c: In function 'maxval_r4':
.../libgfortran/libgfortran.h:292:30: warning: target format does not support infinity
  292 | # define GFC_REAL_4_INFINITY __builtin_inff ()
      |                              ^~~~~~~~~~~~~~
.../libgfortran/generated/maxval_r4.c:149:19:
note: in expansion of macro 'GFC_REAL_4_INFINITY'
  149 |         result = -GFC_REAL_4_INFINITY;
      |                   ^~~~~~~~~~~~~~~~~~~
.../libgfortran/generated/maxval_r4.c: In function 'mmaxval_r4':
.../libgfortran/libgfortran.h:292:30: warning: target format does not support infinity
  292 | # define GFC_REAL_4_INFINITY __builtin_inff ()
      |                              ^~~~~~~~~~~~~~
.../libgfortran/generated/maxval_r4.c:363:19:
note: in expansion of macro 'GFC_REAL_4_INFINITY'
  363 |         result = -GFC_REAL_4_INFINITY;
      |                   ^~~~~~~~~~~~~~~~~~~
{standard input}: Assembler messages:
{standard input}:204: Fatal error: Can't relocate expression
make[3]: *** [Makefile:3358: maxval_r4.lo] Error 1

Correct the checks then for __FLT_HAS_INFINITY__, __DBL_HAS_INFINITY__,
__LDBL_HAS_INFINITY__, __FLT_HAS_QUIET_NAN__, __DBL_HAS_QUIET_NAN__, and
__LDBL_HAS_QUIET_NAN__ to match semantics and remove build issues coming
from the misinterpretation of these macros.

	libgfortran/
	* libgfortran.h: Use #if rather than #ifdef with
	__FLT_HAS_INFINITY__, __DBL_HAS_INFINITY__,
	__LDBL_HAS_INFINITY__, __FLT_HAS_QUIET_NAN__,
	__DBL_HAS_QUIET_NAN__, and __LDBL_HAS_QUIET_NAN__.