Remove references to _DECLARE_C99_LDBL_MATH.

When building openlibm with Clang, I seem to get a lot of warnings in
ld80/ related to some prototypes for long double functions that are
missing. This seems to be because we don't define _DECLARE_C99_LDBL_MATH
anywhere.

It seems that this definition only existed on FreeBSD, as certain C99
math functions were not present yet. The prototypes were simply there as
placeholders. This flag has been removed upstream (FreeBSD SVN r236148).

src/openlibm.h

@@ -499,35 +499,23 @@ float	significandf(float);
  * long double versions of ISO/POSIX math functions
  */
 #if __ISO_C_VISIBLE >= 1999
-#if _DECLARE_C99_LDBL_MATH
 long double	acoshl(long double);
-#endif
 long double	acosl(long double);
-#if _DECLARE_C99_LDBL_MATH
 long double	asinhl(long double);
-#endif
 long double	asinl(long double);
 long double	atan2l(long double, long double);
-#if _DECLARE_C99_LDBL_MATH
 long double	atanhl(long double);
-#endif
 long double	atanl(long double);
 long double	cbrtl(long double);
 long double	ceill(long double);
 long double	copysignl(long double, long double) __pure2;
-#if _DECLARE_C99_LDBL_MATH
 long double	coshl(long double);
-#endif
 long double	cosl(long double);
-#if _DECLARE_C99_LDBL_MATH
 long double	erfcl(long double);
 long double	erfl(long double);
-#endif
 long double	exp2l(long double);
-#if _DECLARE_C99_LDBL_MATH
 long double	expl(long double);
 long double	expm1l(long double);
-#endif
 long double	fabsl(long double) __pure2;
 long double	fdiml(long double, long double);
 long double	floorl(long double);
@@ -539,20 +527,14 @@ long double	frexpl(long double value, int *); /* fundamentally !__pure2 */
 long double	hypotl(long double, long double);
 int		ilogbl(long double) __pure2;
 long double	ldexpl(long double, int);
-#if _DECLARE_C99_LDBL_MATH
 long double	lgammal(long double);
-#endif
 long long	llrintl(long double);
 long long	llroundl(long double);
-#if _DECLARE_C99_LDBL_MATH
 long double	log10l(long double);
 long double	log1pl(long double);
 long double	log2l(long double);
-#endif
 long double	logbl(long double);
-#if _DECLARE_C99_LDBL_MATH
 long double	logl(long double);
-#endif
 long		lrintl(long double);
 long		lroundl(long double);
 long double	modfl(long double, long double *); /* fundamentally !__pure2 */
@@ -562,27 +544,19 @@ long double	nextafterl(long double, long double);
 double		nexttoward(double, long double);
 float		nexttowardf(float, long double);
 long double	nexttowardl(long double, long double);
-#if _DECLARE_C99_LDBL_MATH
 long double	powl(long double, long double);
-#endif
 long double	remainderl(long double, long double);
 long double	remquol(long double, long double, int *);
 long double	rintl(long double);
 long double	roundl(long double);
 long double	scalblnl(long double, long);
 long double	scalbnl(long double, int);
-#if _DECLARE_C99_LDBL_MATH
 long double	sinhl(long double);
-#endif
 long double	sinl(long double);
 long double	sqrtl(long double);
-#if _DECLARE_C99_LDBL_MATH
 long double	tanhl(long double);
-#endif
 long double	tanl(long double);
-#if _DECLARE_C99_LDBL_MATH
 long double	tgammal(long double);
-#endif
 long double	truncl(long double);
 
 #endif /* __ISO_C_VISIBLE >= 1999 */