Linux: incremental build speedup (#10147)

* qgroundcontrol.pro: use gold linker if available (on linux)

Speeds up linking by ~4s on my machine.

* linux-fixup-rpaths.bash: use timestamp file to avoid unnecessary re-evaluation

Speeds up incremental builds, this step takes ~17s.

QGCPostLinkCommon.pri

@@ -130,9 +130,9 @@ LinuxBuild {
             libicui18n.so \
             libicuuc.so
     }
-
+    # Copy only if non-existing to avoid file timestamp updates
     for(QT_LIB, QT_LIB_LIST) {
-        QMAKE_POST_LINK += && $$QMAKE_COPY --dereference $$[QT_INSTALL_LIBS]/$$QT_LIB $$DESTDIR/Qt/libs/
+        QMAKE_POST_LINK += && $$QMAKE_COPY -n --dereference $$[QT_INSTALL_LIBS]/$$QT_LIB $$DESTDIR/Qt/libs/
     }
 
     # QT_INSTALL_PLUGINS
@@ -152,11 +152,11 @@ LinuxBuild {
     }
 
     for(QT_PLUGIN, QT_PLUGIN_LIST) {
-        QMAKE_POST_LINK += && $$QMAKE_COPY --dereference --recursive $$[QT_INSTALL_PLUGINS]/$$QT_PLUGIN $$DESTDIR/Qt/plugins/
+        QMAKE_POST_LINK += && $$QMAKE_COPY -n --dereference --recursive $$[QT_INSTALL_PLUGINS]/$$QT_PLUGIN $$DESTDIR/Qt/plugins/
     }
 
     # QT_INSTALL_QML
-    QMAKE_POST_LINK += && $$QMAKE_COPY --dereference --recursive $$[QT_INSTALL_QML] $$DESTDIR/Qt/
+    QMAKE_POST_LINK += && $$QMAKE_COPY -n --dereference --recursive $$[QT_INSTALL_QML] $$DESTDIR/Qt/
 
     # Airmap
     contains (DEFINES, QGC_AIRMAP_ENABLED) {

deploy/linux-fixup-rpaths.bash

@@ -47,6 +47,10 @@ find "${SEARCHDIR}" \
     -executable \
     2>/dev/null |
 while IFS='' read -r library; do
+
+    # readelf is expensive, so keep track of updates with a timestamp file
+    if [ ! -e "$library.stamp" -o "$library" -nt "$library.stamp" ]; then
+
         # Get the library's current RPATH (RUNPATH)
         # Example output of `readelf -d ./build/build-qgroundcontrol-Desktop_Qt_5_15_2_GCC_64bit-Debug/staging/QGroundControl`:
         #  0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN/Qt/libs:/home/kbennett/storage/Qt/5.15.2/gcc_64/lib]
@@ -83,4 +87,7 @@ while IFS='' read -r library; do
 
         # patch the library's rpath
         patchelf --set-rpath "${new_rpath}" "${library}"
+
+        touch "$library.stamp"
+    fi
 done

qgroundcontrol.pro

@@ -46,6 +46,9 @@ MacBuild {
 
 LinuxBuild {
     CONFIG += qesp_linux_udev
+    system("$$QMAKE_LINK -fuse-ld=gold -Wl,--version &>/dev/null") {
+        CONFIG += use_gold_linker
+    }
 }
 
 WindowsBuild {