[flang] Tighten requirements on some glibc float128 functions (llvm#110651)

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/lib/Evaluate/intrinsics-library.cpp

@@ -417,7 +417,7 @@ template <> struct HostRuntimeLibrary<double, LibraryVersion::LibmExtensions> {
   static_assert(map.Verify(), "map must be sorted");
 };
 
-#if HAS_FLOAT80 || HAS_LDBL128
+#if defined(__GLIBC__) && (HAS_FLOAT80 || HAS_LDBL128)
 template <>
 struct HostRuntimeLibrary<long double, LibraryVersion::LibmExtensions> {
   using F = FuncPointer<long double, long double>;

flang/runtime/Float128Math/math-entries.h

@@ -187,9 +187,6 @@ 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)
@@ -207,9 +204,15 @@ 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 \