ni845x_spi: Add a `ni845x.h` header stub for build tests

The `ni845x_spi' programmer driver depends on a proprietary library,
only available for Windows. As that's not the best circumstances for
build tests, add a header file that provides exactly the definitions
needed to compile the driver.

The compilation result can never be linked. Hence, add another build
target to test compilation of this file only.

Change-Id: I3d291bc16eecb1777062a61362a8eb1548afb7a3
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/411
diff --git a/Makefile b/Makefile
index 4810c34..9c0a336 100644
--- a/Makefile
+++ b/Makefile
@@ -1026,6 +1026,11 @@
 strip: $(PROGRAM)$(EXEC_SUFFIX)
 	$(STRIP) $(STRIP_ARGS) $(PROGRAM)$(EXEC_SUFFIX)
 
+additional_tests: CPPFLAGS += -Iinclude/test
+ifeq ($(CONFIG_NI845X_SPI), no)
+additional_tests: ni845x_spi.o
+endif
+
 # Make sure to add all names of generated binaries here.
 # This includes all frontends and libflashprog.
 # We don't use EXEC_SUFFIX here because we want to clean everything.
@@ -1116,7 +1121,8 @@
 gitconfig:
 	./util/getrevision.sh -c 2>/dev/null && ./util/git-hooks/install.sh
 
-.PHONY: all install clean distclean config branch tag versioninfo _export export tarball libpayload gitconfig
+.PHONY: all install clean distclean config additional_tests branch tag
+.PHONY: versioninfo _export export tarball libpayload gitconfig
 
 # Disable implicit suffixes and built-in rules (for performance and profit)
 .SUFFIXES: