The FX build uses ant to build the FX apps -- "gradle apps" exec's ant and runs that in the apps dir.

The recently released ant 1.10.7 release fails with the following error:

$ gradle apps
-do-compile:
    [mkdir] Created dir: /home/username/jfx/rt/apps/samples/3DViewer/build/empty
    [mkdir] Created dir: /home/username/jfx/rt/apps/samples/3DViewer/build/generated-sources/ap-source-output
    [javac] Compiling 143 source files to /home/username/jfx/rt/apps/samples/3DViewer/build/classes
    [javac] error: invalid flag: @/home/username/jfx/rt/build/compile.args
    [javac] Usage: javac <options> <source files>
    [javac] use --help for a list of possible options

BUILD FAILED

Here is the version info from ant:

$ ant -version
Apache Ant(TM) version 1.10.7 compiled on September 1 2019

This is almost certainly an ant bug, so I plan to file a bug against Apache ant. This bug will be left as a placeholder, and eventually closed as "External".