Fix AppImage build: need LD_LIBRARY_PATH

I'm not sure why this is needed now and wasn't needed before since the
cmake changes have been in main for a while and there don't appear to
be recent changes to the appimage build tools.

This change adds to lib directory to LD_LIBRARY_PATH so the appimage
tools' ldd check will pass. This is an appropriate change since the
directory that is added will be in the standard load path when
installed.

README-maintainer

@@ -443,6 +443,16 @@ build-appimage, which passes them along to to docker.
 Use -e SKIP_TESTS=1 to skip the test suite.
 Use -ti -e RUN_SHELL=1 to run a shell instead of the build script.
 
+To iterate on the scripts directly in the source tree, you can run
+
+docker build -t qpdfbuild appimage
+docker run --privileged --rm -ti -e SKIP_TESTS=1 -e RUN_SHELL=1 \
+       -v $PWD/..:/tmp/build ${1+"$@"} qpdfbuild
+
+This will put you at a shell prompt inside the container with your
+current directory set to the top of the source tree and your uid equal
+to the owner of the parent directory source tree.
+
 
 LOCAL WINDOWS TESTING PROCEDURE
 

appimage/build-appimage

@@ -21,7 +21,7 @@
 # into the 'CUSTOM_CONFIGURE' environment variable and exporting it
 # before running this script. For example:
 #
-#      export CUSTOM_CONFIGURE=" --enable-test-compare-images [--more-other-options]"
+#      export CUSTOM_CONFIGURE=" -DSOME_OTHER_OPTION=1 [...]"
 #
 # ATTENTION:
 #
@@ -151,7 +151,12 @@ wget -c "https://github.com/probonopd/linuxdeployqt/releases/download/continuous
 chmod a+x linuxdeployqt*.AppImage
 
 # Set up a clean environment:
-unset QTDIR; unset QT_PLUGIN_PATH; unset LD_LIBRARY_PATH
+unset QTDIR; unset QT_PLUGIN_PATH
+
+# Since cmake strips runpath information for executables and we are
+# installing with DESTDIR, we need to set LD_LIBRARY_PATH or else ldd
+# will fail on the resulting executables
+export LD_LIBRARY_PATH=$(dirname $(find $appdir -type l -name libqpdf.so.28))
 
 # Let 'linuxdeployqt' do its first stage of work:
 ./linuxdeployqt*.AppImage appdir/usr/share/applications/*.desktop -bundle-non-qt-libs