Index: include/clang/Basic/BuiltinsWebAssembly.def
===================================================================
--- include/clang/Basic/BuiltinsWebAssembly.def
+++ include/clang/Basic/BuiltinsWebAssembly.def
@@ -16,9 +16,21 @@
 
 // The format of this database matches clang/Basic/Builtins.def.
 
+#if defined(BUILTIN) && !defined(LIBBUILTIN)
+#  define LIBBUILTIN(ID, TYPE, ATTRS, HEADER) BUILTIN(ID, TYPE, ATTRS)
+#endif
+
 // Note that current_memory is not "c" (readnone) because it must be sequenced
 // with respect to grow_memory calls.
 BUILTIN(__builtin_wasm_current_memory, "z", "n")
 BUILTIN(__builtin_wasm_grow_memory, "zz", "n")
 
+// Redeclaring lgamma from Builtins.def as non-const to allow for POSIX
+// compatibility. Specifically lgamma writes to a user-visible global, signgam,
+// that is available in POSIX-compliant libcs.
+LIBBUILTIN(lgamma, "dd", "fn", "math.h")
+LIBBUILTIN(lgammaf, "ff", "fn", "math.h")
+LIBBUILTIN(lgammal, "LdLd", "fn", "math.h")
+
 #undef BUILTIN
+#undef LIBBUILTIN