unclutter: fix build (#404065)

pkgs/by-name/un/unclutter/package.nix

@@ -15,13 +15,24 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ libX11 ];
 
-  buildFlags = [ "CC=${stdenv.cc.targetPrefix}cc" ];
+  buildFlags = [
+    "CC=${stdenv.cc.targetPrefix}cc"
+    "CFLAGS=-std=c89"
+  ];
 
-  installPhase = ''
-    mkdir -pv "$out/bin"
-    mkdir -pv "$out/share/man/man1"
-    make DESTDIR="$out" BINDIR="$out/bin" PREFIX="" install
-    make DESTDIR="$out" MANPATH="$out/share/man" PREFIX="" install.man
+  installFlags = [
+    "DESTDIR=${placeholder "out"}"
+    "BINDIR=${placeholder "out"}/bin"
+    "MANPATH=${placeholder "out"}/share/man"
+  ];
+
+  installTargets = [
+    "install"
+    "install.man"
+  ];
+
+  preInstall = ''
+    mkdir -pv "$out"/{bin,share/man/man1}
   '';
 
   meta = with lib; {