I changed
LIBS += $$join(SECP256K1_LIB_PATH,,-L,) -lsecp256k1
to
LIBS += $$join(SECP256K1_LIB_PATH,,-L,) -L/mnt/COINNAME/src/secp256k1
and got past
/mnt/mxe/usr/bin/i686-w64-mingw32.static-ld: cannot find -lsecp256k1
There's other errors now so I'm not sure if it's the proper fix but it might help some people.