meson: Configure with -Db_lundef=false on FreeBSD

Meson sets a default for all platforms which is -Db_lundef=true, meaning
always add the --no-undefined flag to linker invocations. This is a
consistently bad idea on FreeBSD, since the `environ` variable is
provided not by libc but by the csu, leading to the linker complaining
whenever it is used.

pkgs/by-name/gl/glib/package.nix

@@ -249,7 +249,6 @@ stdenv.mkDerivation (finalAttrs: {
       "-Dlibelf=disabled"
     ]
     ++ lib.optionals stdenv.hostPlatform.isFreeBSD [
-      "-Db_lundef=false"
       "-Dxattr=false"
       "-Dsysprof=disabled" # sysprof-capture does not build on FreeBSD
     ];

pkgs/by-name/me/meson/package.nix

@@ -144,6 +144,7 @@ python3.pkgs.buildPythonApplication rec {
   '';
 
   setupHook = ./setup-hook.sh;
+  env.hostPlatform = stdenv.targetPlatform.system;
 
   meta = {
     homepage = "https://mesonbuild.com";

pkgs/by-name/me/meson/setup-hook.sh

@@ -26,6 +26,11 @@ mesonConfigurePhase() {
         "--buildtype=${mesonBuildType:-plain}"
     )
 
+    # --no-undefined is universally a bad idea on freebsd because environ is in the csu
+    if [[ "@hostPlatform@" == *-freebsd ]]; then
+        flagsArray+=("-Db_lundef=false")
+    fi
+
     concatTo flagsArray mesonFlags mesonFlagsArray
 
     echoCmd 'mesonConfigurePhase flags' "${flagsArray[@]}"