cmake: only set output paths with multiple outputs

This brings cmake inline with the behaviour used for configure
scripts, defined in multiple-outputs.sh.  It's important because
that setup hook will only set shareDocName if multiple outputs are in
use (and setOutputFlags hasn't been disabled).  So previously,
CMAKE_INSTALL_DOCDIR would be set to $out/share/doc for single-output
derivations, instead of $out/share/doc/$shareDocName, which would
result in collisions.

Since this hook now uses the setOutputFlags variable, I had to remove
the empty assignment of it added in
a714284.

Fixes: NixOS#82304

pkgs/development/tools/build-managers/cmake/setup-hook.sh

@@ -68,19 +68,24 @@ cmakeConfigurePhase() {
     # nix/store directory.
     cmakeFlags="-DCMAKE_INSTALL_NAME_DIR=${!outputLib}/lib $cmakeFlags"
 
-    # This ensures correct paths with multiple output derivations
-    # It requires the project to use variables from GNUInstallDirs module
-    # https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
-    cmakeFlags="-DCMAKE_INSTALL_BINDIR=${!outputBin}/bin $cmakeFlags"
-    cmakeFlags="-DCMAKE_INSTALL_SBINDIR=${!outputBin}/sbin $cmakeFlags"
-    cmakeFlags="-DCMAKE_INSTALL_INCLUDEDIR=${!outputInclude}/include $cmakeFlags"
-    cmakeFlags="-DCMAKE_INSTALL_OLDINCLUDEDIR=${!outputInclude}/include $cmakeFlags"
-    cmakeFlags="-DCMAKE_INSTALL_MANDIR=${!outputMan}/share/man $cmakeFlags"
-    cmakeFlags="-DCMAKE_INSTALL_INFODIR=${!outputInfo}/share/info $cmakeFlags"
-    cmakeFlags="-DCMAKE_INSTALL_DOCDIR=${!outputDoc}/share/doc/${shareDocName} $cmakeFlags"
+    if [ "$outputs" != "out" -a -n "${setOutputFlags-1}" ]; then
+        # This ensures correct paths with multiple output derivations
+        # It requires the project to use variables from GNUInstallDirs module
+        # https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
+        cmakeFlags="-DCMAKE_INSTALL_BINDIR=${!outputBin}/bin $cmakeFlags"
+        cmakeFlags="-DCMAKE_INSTALL_SBINDIR=${!outputBin}/sbin $cmakeFlags"
+        cmakeFlags="-DCMAKE_INSTALL_INCLUDEDIR=${!outputInclude}/include $cmakeFlags"
+        cmakeFlags="-DCMAKE_INSTALL_OLDINCLUDEDIR=${!outputInclude}/include $cmakeFlags"
+        cmakeFlags="-DCMAKE_INSTALL_MANDIR=${!outputMan}/share/man $cmakeFlags"
+        cmakeFlags="-DCMAKE_INSTALL_INFODIR=${!outputInfo}/share/info $cmakeFlags"
+        cmakeFlags="-DCMAKE_INSTALL_DOCDIR=${!outputDoc}/share/doc/${shareDocName} $cmakeFlags"
+        cmakeFlags="-DCMAKE_INSTALL_LIBEXECDIR=${!outputLib}/libexec $cmakeFlags"
+        cmakeFlags="-DCMAKE_INSTALL_LOCALEDIR=${!outputLib}/share/locale $cmakeFlags"
+    fi
+
+    # This output flag must always be set, unlike the others, because
+    # otherwise we end up with lib64.
     cmakeFlags="-DCMAKE_INSTALL_LIBDIR=${!outputLib}/lib $cmakeFlags"
-    cmakeFlags="-DCMAKE_INSTALL_LIBEXECDIR=${!outputLib}/libexec $cmakeFlags"
-    cmakeFlags="-DCMAKE_INSTALL_LOCALEDIR=${!outputLib}/share/locale $cmakeFlags"
 
     # Don’t build tests when doCheck = false
     if [ -z "${doCheck-}" ]; then
@@ -115,7 +120,6 @@ cmakeConfigurePhase() {
 }
 
 if [ -z "${dontUseCmakeConfigure-}" -a -z "${configurePhase-}" ]; then
-    setOutputFlags=
     configurePhase=cmakeConfigurePhase
 fi