build: Add "crosstest" target for tests cross-compilation.

Makefile.am

@@ -44,3 +44,50 @@ libvkd3d.pc: $(srcdir)/libs/vkd3d/libvkd3d.pc.in
 		-e 's![@]libdir[@]!$(libdir)!g' \
 		-e 's![@]PACKAGE_VERSION[@]!$(PACKAGE_VERSION)!g' \
 		$< > $@
+
+## Cross-compile tests
+cross_implibs = crosslibs/d3d12
+CROSS_CPPFLAGS = -I$(srcdir)/include -I$(srcdir)/include/private -I$(builddir)/include
+
+if HAS_CROSSTARGET32
+CROSS32_CC = @CROSSCC32@
+CROSS32_DLLTOOL = @CROSSTARGET32@-dlltool
+CROSS32_IMPLIBS = $(cross_implibs:=.cross32.a)
+CROSS32_EXEFILES = $(vkd3d_tests:=.cross32.exe)
+CROSS32_FILES = $(CROSS32_IMPLIBS) $(CROSS32_EXEFILES)
+
+CLEANFILES += $(CROSS32_FILES)
+crosstest32: $(widl_headers) $(CROSS32_FILES)
+
+$(CROSS32_IMPLIBS): %.cross32.a: %.cross32.def
+	${MKDIR_P} crosslibs
+	$(CROSS32_DLLTOOL) -k -m i386 --as-flags=-32 -d $< -l $@
+
+$(CROSS32_EXEFILES): %.cross32.exe: %.c $(CROSS32_IMPLIBS)
+	$(CROSS32_CC) $(CROSS_CPPFLAGS) -o $@ $< $(CROSS32_IMPLIBS)
+else
+crosstest32:
+endif
+
+if HAS_CROSSTARGET64
+CROSS64_CC = @CROSSCC64@
+CROSS64_DLLTOOL = @CROSSTARGET64@-dlltool
+CROSS64_IMPLIBS = $(cross_implibs:=.cross64.a)
+CROSS64_EXEFILES = $(vkd3d_tests:=.cross64.exe)
+CROSS64_FILES = $(CROSS64_IMPLIBS) $(CROSS64_EXEFILES)
+
+CLEANFILES += $(CROSS64_FILES)
+crosstest64: $(widl_headers) $(CROSS64_FILES)
+
+$(CROSS64_IMPLIBS): %.cross64.a: %.cross64.def
+	${MKDIR_P} crosslibs
+	$(CROSS64_DLLTOOL) -k -m i386:x86-64 --as-flags=-64 -d $< -l $@
+
+$(CROSS64_EXEFILES): %.cross64.exe: %.c $(CROSS64_IMPLIBS)
+	$(CROSS64_CC) $(CROSS_CPPFLAGS) -o $@ $< $(CROSS64_IMPLIBS)
+else
+crosstest64:
+endif
+
+.PHONY: crosstest crosstest32 crosstest64
+crosstest: crosstest32 crosstest64