Qt with Xcode 11.4 no longer creates valid dylibs

Xcode recently updated to 11.4, and now the dylibs that are produced from qmake are missing the LC_VERSION_MIN_MACOSX SDK version:

matt@host build-TestLibrary-Desktop_Qt_5_12_7_clang_64bit-Release % otool -l libTestLibrary.dylib| grep LC_VERSION_MIN_MACOSX -A 3

cmd LC_VERSION_MIN_MACOSX
cmdsize 16
version 10.12
sdk n/a

sdk should not be "n/a", but rather the version of the SDK the library was compiled against. If it's missing, hardened runtime library validation will fail and apps that depend on the library can't be run. A workaround is to add 

QMAKE_LFLAGS += -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk 

to the .pro file which then produces a correct binary:

cmd LC_VERSION_MIN_MACOSX
cmdsize 16
version 10.12
sdk 10.15.4

and library validation then works as expected.