Remove -Wno-unknown-pragmas from fusl build

fusl/src/internal/libm.h

 /* origin: FreeBSD /usr/src/lib/msun/src/math_private.h */
 /*
  * ====================================================
  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
  *
  * Developed at SunPro, a Sun Microsystems, Inc. business.
  * Permission to use, copy, modify, and distribute this
  * software is freely granted, provided that this notice
  * is preserved.
  * ====================================================
  */
 
 #ifndef _LIBM_H
 #define _LIBM_H
 
 #include <stdint.h>
 #include <float.h>
 #include <math.h>
 #include <complex.h>
 #include <endian.h>
 
+/* Clang does not support the C99 rounding mode pragma. Support seems
+ * unlikely to be coming soon, but for reference the clang/llvm bug
+ * tracking this fact may be found at:
+ * https://llvm.org/bugs/show_bug.cgi?id=8100 */
+#ifdef __clang__
+#define PRAGMA_STDC_FENV_ACCESS_ON
+#else
+#define PRAGMA_STDC_FENV_ACCESS_ON _Pragma("STDC FENV_ACCESS ON")
+#endif
+
 #if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
 #elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384 && __BYTE_ORDER == __LITTLE_ENDIAN
 union ldshape {
         long double f;
         struct {
                 uint64_t m;
                 uint16_t se;
         } i;
 };
 #elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384 && __BYTE_ORDER == __LITTLE_ENDIAN
@@ skipping 143 matching lines @@
 int __rem_pio2l(long double, long double *);
 long double __sinl(long double, long double, int);
 long double __cosl(long double, long double);
 long double __tanl(long double, long double, int);
 
 /* polynomial evaluation */
 long double __polevll(long double, const long double *, int);
 long double __p1evll(long double, const long double *, int);
 
 #endif