diff --git a/pkgs/development/libraries/cairo/default.nix b/pkgs/development/libraries/cairo/default.nix
index cdf994442d372..8ed46ae3efb50 100644
--- a/pkgs/development/libraries/cairo/default.nix
+++ b/pkgs/development/libraries/cairo/default.nix
@@ -87,7 +87,7 @@ in {
     # `-I' flags to be propagated.
     sed -i "$out/lib/pkgconfig/cairo.pc" \
         -es'|^Cflags:\(.*\)$|Cflags: \1 -I${freetype.dev}/include/freetype2 -I${freetype.dev}/include|g'
-  '' + lib.optionalString stdenv.hostPlatform.isDarwin glib.flattenInclude;
+  '';
 
   passthru.tests.pkg-config = testers.testMetaPkgConfig finalAttrs.finalPackage;
 
diff --git a/pkgs/development/libraries/freetype/default.nix b/pkgs/development/libraries/freetype/default.nix
index 814224a44fbe8..1586ce27e69f3 100644
--- a/pkgs/development/libraries/freetype/default.nix
+++ b/pkgs/development/libraries/freetype/default.nix
@@ -72,10 +72,9 @@ stdenv.mkDerivation (finalAttrs: {
 
   doCheck = true;
 
-  postInstall = glib.flattenInclude
-    # pkgsCross.mingwW64.pkg-config doesn't build
-    # makeWrapper doesn't cross-compile to windows #120726
-    + ''
+  # pkgsCross.mingwW64.pkg-config doesn't build
+  # makeWrapper doesn't cross-compile to windows #120726
+  postInstall = ''
     substituteInPlace $dev/bin/freetype-config \
       --replace ${buildPackages.pkg-config} ${pkgsHostHost.pkg-config}
   '' + lib.optionalString (!stdenv.hostPlatform.isMinGW) ''
diff --git a/pkgs/development/libraries/glib/default.nix b/pkgs/development/libraries/glib/default.nix
index f44b6385b8f1f..eda23120af8c9 100644
--- a/pkgs/development/libraries/glib/default.nix
+++ b/pkgs/development/libraries/glib/default.nix
@@ -33,19 +33,6 @@
 assert stdenv.hostPlatform.isLinux -> util-linuxMinimal != null;
 
 let
-  # Some packages don't get "Cflags" from pkg-config correctly
-  # and then fail to build when directly including like <glib/...>.
-  # This is intended to be run in postInstall of any package
-  # which has $out/include/ containing just some disjunct directories.
-  flattenInclude = ''
-    for dir in "''${!outputInclude}"/include/*; do
-      cp -r "$dir"/* "''${!outputInclude}/include/"
-      rm -r "$dir"
-      ln -s . "$dir"
-    done
-    ln -sr -t "''${!outputInclude}/include/" "''${!outputInclude}"/lib/*/include/* 2>/dev/null || true
-  '';
-
   gobject-introspection' = buildPackages.gobject-introspection.override {
     propagateFullGlib = false;
     # Avoid introducing cairo, which enables gobjectSupport by default.
@@ -306,7 +293,6 @@ stdenv.mkDerivation (finalAttrs: {
       pkg-config = testers.testMetaPkgConfig finalAttrs.finalPackage;
     };
 
-    inherit flattenInclude;
     updateScript = gnome.updateScript {
       packageName = "glib";
       versionPolicy = "odd-unstable";
diff --git a/pkgs/development/libraries/pixman/default.nix b/pkgs/development/libraries/pixman/default.nix
index dbf742ab81f25..b5bdcec35e562 100644
--- a/pkgs/development/libraries/pixman/default.nix
+++ b/pkgs/development/libraries/pixman/default.nix
@@ -63,8 +63,6 @@ stdenv.mkDerivation (finalAttrs: {
 
   doCheck = !stdenv.hostPlatform.isDarwin;
 
-  postInstall = glib.flattenInclude;
-
   passthru = {
     tests = {
       inherit cairo qemu scribus tigervnc wlroots_0_17 wlroots_0_18 xwayland;