Building on Mac with prebuilt ffmpeg libraries fails with Xcode 12.1

[jg18]

When I run xcodebuild -configuration Release I get this output at the end:

CodeSign build/bin/Release/fs2_open_20_1_0_x64.app
    cd /path/to/fs2open
    export CODESIGN_ALLOCATE=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/codesign_allocate
    
Signing Identity:     "-"

    /usr/bin/codesign --force --sign - --entitlements /path/to/fs2open/build/freespace2/FS2_Open.build/Release/Freespace2.build/fs2_open_20_1_0_x64.app.xcent --timestamp=none /path/to/fs2open/build/bin/Release/fs2_open_20_1_0_x64.app
/path/to/fs2open/build/bin/Release/fs2_open_20_1_0_x64.app: code object is not signed at all
In subcomponent: /path/to/fs2open/build/bin/Release/fs2_open_20_1_0_x64.app/Contents/Frameworks/libswresample.2.1.100.dylib
Command /usr/bin/codesign failed with exit code 1

** BUILD FAILED **


The following build commands failed:
	CodeSign build/bin/Release/fs2_open_20_1_0_x64.app
(1 failure)

As a workaround, when I run the codesign command on all of the ffmpeg dylib files, such as with

for i in `ls -1 *.dylib`; do echo $i; /usr/bin/codesign --force --sign - --entitlements /path/to/fs2open/build/freespace2/FS2_Open.build/FastDebug/Freespace2.build/fs2_open_20_1_0_x64-FASTDBG.app.xcent  $i; done

then it works. It looks like some .dylibs are already signed and some aren't. It might be good to have them all signed by the same cert.

Using macOS 10.15.4 with Xcode 12.1 (Build version 12A7403).