I've got it... I needed to disable all the AVX stuff.

Presumably, this is broken because I cross-compile.

src/syncterm/GNUmakefile

@@ -36,7 +36,6 @@ else
 endif
 
 ifdef win
- ifeq (${os}, "win32")
  CFLAGS += -I${3RDP_ROOT}/win32.release/libjxl/include -DJXL_THREADS_STATIC_DEFINE -DJXL_STATIC_DEFINE
  LDFLAGS += -L${3RDP_ROOT}/${os}.release/libjxl/lib -static-libstdc++ -static-libgcc
  EXTRA_LIBS += -ljxl
@@ -47,7 +46,6 @@ ifdef win
  CFLAGS += -DWITH_JPEG_XL -DWITH_STATIC_JXL
  CFLAGS += -DWITH_JPEG_XL_THREADS
  OBJS += $(MTOBJODIR)$(DIRSEP)libjxl$(OFILE)
- endif
 else
  ifeq ($(shell pkg-config libjxl --exists && echo YES), YES)
   CFLAGS += $(shell pkg-config libjxl --cflags)