diff --git a/build.sh b/build.sh
index 24a8a1e5..44a1d91e 100755
--- a/build.sh
+++ b/build.sh
@@ -4,6 +4,7 @@ set -e
 
 QT_CFG=''
 
+BUILD_CONFIRM=0
 COMPILE_JOBS=1
 MAKEFLAGS_JOBS=''
 
@@ -43,9 +44,13 @@ until [ -z "$1" ]; do
             shift
             COMPILE_JOBS=$1
             shift;;
+        "--confirm")
+            BUILD_CONFIRM=1
+            shift;;
         "--help")
             echo "Usage: $0 [--qt-config CONFIG] [--jobs NUM]"
             echo
+            echo "  --confirm                   Silently confirm the build."
             echo "  --qt-config CONFIG          Specify extra config options to be used when configuring Qt"
             echo "  --jobs NUM                  How many parallel compile jobs to use. Defaults to 4."
             echo
@@ -57,6 +62,30 @@ until [ -z "$1" ]; do
     esac
 done
 
+
+if [[ "$BUILD_CONFIRM" -eq 0 ]]; then
+cat << EOF
+----------------------------------------
+               WARNING
+----------------------------------------
+
+Building PhantomJS from source takes a very long time, anywhere from 30 minutes
+to several hours (depending on the machine configuration). It is recommended to
+use the premade binary packages on supported operating systems.
+
+For details, please go the the web site: http://phantomjs.org/download.html.
+
+EOF
+
+    echo "Do you want to continue (y/n)?"
+    read continue
+    if [[ "$continue" != "y" ]]; then
+        exit 1
+    fi
+    echo
+    echo
+fi
+
 cd src/qt && ./preconfig.sh --jobs $COMPILE_JOBS --qt-config "$QT_CFG" && cd ../..
 src/qt/bin/qmake $QMAKE_ARGS
 make -j$COMPILE_JOBS
diff --git a/deploy/build-and-package.sh b/deploy/build-and-package.sh
index 8b7fa88e..14691f3f 100755
--- a/deploy/build-and-package.sh
+++ b/deploy/build-and-package.sh
@@ -11,7 +11,7 @@ echo
 # This incantation will cause Qt and WebKit and PhantomJS to all build in "release"
 # mode, with compiler optimisations, but also with debug symbols. (We will strip the
 # symbols in package.sh.)
-CFLAGS=-g CXXFLAGS=-g ./build.sh --qt-config '-webkit-debug' --qmake-args "QMAKE_CFLAGS=-g QMAKE_CXXFLAGS=-g" || exit 1
+CFLAGS=-g CXXFLAGS=-g ./build.sh --confirm --qt-config '-webkit-debug' --qmake-args "QMAKE_CFLAGS=-g QMAKE_CXXFLAGS=-g" || exit 1
 
 # Package the release tarball
 rm deploy/*.tar.bz2 2>/dev/null