lcalc: use libc++-compatible way of extending namespace std (#370545)

pkgs/by-name/lc/lcalc/libcxx-compat.patch

@@ -0,0 +1,30 @@
+diff --git a/src/libLfunction/Lcomplex.h b/src/libLfunction/Lcomplex.h
+index 363bbf4..ffecd70 100644
+--- a/src/libLfunction/Lcomplex.h
++++ b/src/libLfunction/Lcomplex.h
+@@ -56,8 +56,11 @@
+ #include <cmath>
+ #include <sstream>
+ 
+-namespace std
+-{
++#ifdef _LIBCPP_VERSION
++_LIBCPP_BEGIN_NAMESPACE_STD
++#else
++namespace std {
++#endif
+   // Forward declarations
+   template<typename _Tp> class complex;
+   template<> class complex<float>;
+@@ -1193,6 +1196,10 @@ namespace std
+   inline
+   complex<long double>::complex(const complex<double>& __z)
+   : _M_value(_ComplexT(__z._M_value)) { }
+-} // namespace std
++#ifdef _LIBCPP_VERSION
++_LIBCPP_END_NAMESPACE_STD
++#else
++}
++#endif
+ 
+ #endif	/* _CPP_COMPLEX */

pkgs/by-name/lc/lcalc/package.nix

@@ -19,13 +19,9 @@ stdenv.mkDerivation rec {
     hash = "sha256-RxWZ7T0I9zV7jUVnL6jV/PxEoU32KY7Q1UsOL5Lonuc=";
   };
 
-  # workaround for vendored GCC 3.5 <complex>
+  # workaround for vendored GCC <complex> on libc++
   # https://gitlab.com/sagemath/lcalc/-/issues/16
-  env.NIX_CFLAGS_COMPILE = toString [
-    "-D_GLIBCXX_COMPLEX"
-    "-D_LIBCPP_COMPLEX"
-    "-D_LIBCPP___FWD_COMPLEX_H"
-  ];
+  patches = [ ./libcxx-compat.patch ];
 
   nativeBuildInputs = [
     autoreconfHook