AppImage build: fix library issues

* Use correct cmake flag to not build static libraries
* Force correct relative RPATH in installed executables

appimage/build-appimage

@@ -88,8 +88,9 @@ cd build.appimage
 cmake -DWERROR=1 -DBUILD_DOC=1 -DCMAKE_BUILD_TYPE=Release \
       -DBUILD_DOC_DIST=1 -DINSTALL_MANUAL=1 \
       -DREQUIRE_CRYPTO_GNUTLS=1 -DUSE_IMPLICIT_CRYPTO=0 \
-      -DSHOW_FAILED_TEST_OUTPUT=1 -DBUILD_STATIC_LIBRARIES=0 \
-      "$CUSTOM_CONFIGURE" ..
+      -DSHOW_FAILED_TEST_OUTPUT=1 -DBUILD_STATIC_LIBS=0 \
+      -DCMAKE_INSTALL_RPATH='$ORIGIN/../lib' \
+      ${CUSTOM_CONFIGURE+"$CUSTOM_CONFIGURE"} ..
 
 # Build!
 cmake --build . -j$(nproc)