[flang] Tighten requirements on some glibc float128 functions

j0l, j1l, jnl, y0l, y1l and ynl are glibc extensions rather than
standard POSIX functions, and so are not available in every Linux libc.
This patch checks if __GLIBC__ and _GNU_SOURCE are defined before using
these functions.

This patch allows the float128 runtime to build with musl libc on Linux.

Author: DavidTruby

flang/runtime/Float128Math/math-entries.h

@@ -185,9 +185,6 @@ DEFINE_SIMPLE_ALIAS(Hypot, std::hypot)
 DEFINE_SIMPLE_ALIAS(Ilogb, std::ilogb)
 DEFINE_SIMPLE_ALIAS(Isinf, std::isinf)
 DEFINE_SIMPLE_ALIAS(Isnan, std::isnan)
-DEFINE_SIMPLE_ALIAS(J0, j0l)
-DEFINE_SIMPLE_ALIAS(J1, j1l)
-DEFINE_SIMPLE_ALIAS(Jn, jnl)
 DEFINE_SIMPLE_ALIAS(Ldexp, std::ldexp)
 DEFINE_SIMPLE_ALIAS(Lgamma, std::lgamma)
 DEFINE_SIMPLE_ALIAS(Llround, std::llround)
@@ -204,9 +201,15 @@ DEFINE_SIMPLE_ALIAS(Tan, std::tan)
 DEFINE_SIMPLE_ALIAS(Tanh, std::tanh)
 DEFINE_SIMPLE_ALIAS(Tgamma, std::tgamma)
 DEFINE_SIMPLE_ALIAS(Trunc, std::trunc)
+
+#if defined(__GLIBC__) && defined(_GNU_SOURCE)
+DEFINE_SIMPLE_ALIAS(J0, j0l)
+DEFINE_SIMPLE_ALIAS(J1, j1l)
+DEFINE_SIMPLE_ALIAS(Jn, jnl)
 DEFINE_SIMPLE_ALIAS(Y0, y0l)
 DEFINE_SIMPLE_ALIAS(Y1, y1l)
 DEFINE_SIMPLE_ALIAS(Yn, ynl)
+#endif
 
 // Use numeric_limits to produce infinity of the right type.
 #define F128_RT_INFINITY \