stdenv/generic/setup.sh: enable parallel installs by default

The primary motivating example is openssl:

Before the change full package build took 1m54s minutes.
After the change full package build takes 59s.

About a 2x speedup.

The difference is visible because openssl builds hundreds of manpages
spawning a perl process per manual in `install` phase. Such a workload
is very easy to parallelize.

Another example would be `autotools`+`libtool` based build system where
install step requires relinking. The more binaries there are to relink
the more gain it will be to do it in parallel.

The change enables parallel installs by default only for buiilds that
already have parallel builds enabled. There is a high chance those build
systems already handle parallelism well but some packages will fail.

Consistently propagated the enableParallelBuilding to:
- cmake (enabled by default, similar to builds)
- ninja (set parallelism explicitly, don't rely on default)
- bmake (enable when requested)
- scons (enable when requested)
- meson (set parallelism explicitly, don't rely on default)
- waf (set parallelism explicitly, don't rely on default)
- qmake-4/5/6 (enable by default, similar to builds)
- xorg (always enable, similar to builds)

pkgs/development/libraries/qt-4.x/4.8/qmake-hook.sh

···
         echo "qmake4Hook: enabled parallel building"
     fi
 
+    if ! [[ -v enableParallelInstalling ]]; then
+        enableParallelInstalling=1
+        echo "qmake: enabled parallel installing"
+    fi
+
     runHook postConfigure
 }
 

pkgs/development/libraries/qt-5/hooks/qmake-hook.sh

···
         echo "qmake: enabled parallel building"
     fi
 
+    if ! [[ -v enableParallelInstalling ]]; then
+        enableParallelInstalling=1
+        echo "qmake: enabled parallel installing"
+    fi
+
     runHook postConfigure
 }
 

pkgs/development/libraries/qt-6/hooks/qmake-hook.sh

···
         echo "qmake: enabled parallel building"
     fi
 
+    if ! [[ -v enableParallelInstalling ]]; then
+        enableParallelInstalling=1
+        echo "qmake: enabled parallel installing"
+    fi
+
     runHook postConfigure
 }
 

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

···
 
     # shellcheck disable=SC2086
     local flagsArray=(
+        ${enableParallelInstalling:+-j${NIX_BUILD_CORES}}
         SHELL=$SHELL
         # Old bash empty array hack
         $makeFlags ${makeFlagsArray+"${makeFlagsArray[@]}"}

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

···
         echo "cmake: enabled parallel building"
     fi
 
+    if ! [[ -v enableParallelInstalling ]]; then
+        enableParallelInstalling=1
+        echo "cmake: enabled parallel installing"
+    fi
+
     runHook postConfigure
 }
 

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

···
         echo "meson: enabled parallel building"
     fi
 
+    if ! [[ -v enableParallelInstalling ]]; then
+        enableParallelInstalling=1
+        echo "meson: enabled parallel installing"
+    fi
+
     runHook postConfigure
 }
 

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

···
 ninjaInstallPhase() {
     runHook preInstall
 
+    local buildCores=1
+
+    # Parallel building is enabled by default.
+    if [ "${enableParallelInstalling-1}" ]; then
+        buildCores="$NIX_BUILD_CORES"
+    fi
+
     # shellcheck disable=SC2086
     local flagsArray=(
+        -j$buildCores
         $ninjaFlags "${ninjaFlagsArray[@]}"
         ${installTargets:-install}
     )

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

···
     fi
 
     local flagsArray=(
+        ${enableParallelInstalling:+-j${NIX_BUILD_CORES}}
         $sconsFlags ${sconsFlagsArray[@]}
         $installFlags ${installFlagsArray[@]}
         ${installTargets:-install}

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

···
         echo "waf: enabled parallel building"
     fi
 
+    if ! [[ -v enableParallelInstalling ]]; then
+        enableParallelInstalling=1
+        echo "waf: enabled parallel installing"
+    fi
+
     runHook postConfigure
 }
 
···
     fi
 
     local flagsArray=(
+        ${enableParallelInstalling:+-j ${NIX_BUILD_CORES}}
         $wafFlags ${wafFlagsArray[@]}
         $installFlags ${installFlagsArray[@]}
         ${installTargets:-install}

pkgs/servers/x11/xorg/builder.sh

···
 
 
 enableParallelBuilding=1
+enableParallelInstalling=1
 
 genericBuild

pkgs/stdenv/generic/make-derivation.nix

···
     } // lib.optionalAttrs (enableParallelBuilding) {
       inherit enableParallelBuilding;
       enableParallelChecking = attrs.enableParallelChecking or true;
+      enableParallelInstalling = attrs.enableParallelInstalling or true;
     } // lib.optionalAttrs (hardeningDisable != [] || hardeningEnable != [] || stdenv.hostPlatform.isMusl) {
       NIX_HARDENING_ENABLE = enabledHardeningOptions;
     } // lib.optionalAttrs (stdenv.hostPlatform.isx86_64 && stdenv.hostPlatform ? gcc.arch) {

pkgs/stdenv/generic/setup.sh

···
 
     # shellcheck disable=SC2086
     local flagsArray=(
+        ${enableParallelInstalling:+-j${NIX_BUILD_CORES}}
         SHELL=$SHELL
     )
     _accumFlagsArray makeFlags makeFlagsArray installFlags installFlagsArray