[libc++] Do not redeclare lgamma_r when targeting the LLVM C library (#102036)

We use lgamma_r for the random normal distribution support. In this
code we redeclare it, which causes issues with the LLVM C library as
this function is marked noexcept in LLVM libc. This patch ensures that
we don't redeclare that function when targeting LLVM libc.

libcxx/include/__configuration/platform.h

@@ -30,15 +30,18 @@
 // ... add new file formats here ...
 #endif
 
-// Need to detect which libc we're using if we're on Linux.
-#if defined(__linux__)
+// To detect which libc we're using
+#if __has_include(<features.h>)
 #  include <features.h>
+#endif
+
+#if defined(__linux__)
 #  if defined(__GLIBC_PREREQ)
 #    define _LIBCPP_GLIBC_PREREQ(a, b) __GLIBC_PREREQ(a, b)
 #  else
 #    define _LIBCPP_GLIBC_PREREQ(a, b) 0
 #  endif // defined(__GLIBC_PREREQ)
-#endif   // defined(__linux__)
+#endif
 
 #ifndef __BYTE_ORDER__
 #  error                                                                                                               \

libcxx/include/__random/binomial_distribution.h

@@ -97,7 +97,8 @@ public:
   }
 };
 
-#ifndef _LIBCPP_MSVCRT_LIKE
+// The LLVM C library provides this with conflicting `noexcept` attributes.
+#if !defined(_LIBCPP_MSVCRT_LIKE) && !defined(__LLVM_LIBC__)
 extern "C" double lgamma_r(double, int*);
 #endif