build: don't use BUILT_SOURCES for the static context header

BUILT_SOURCES simply forces the header to be built first.

Use real dependencies instead. This fixes builds with dependency tracking
disabled where a real target is specified.

As a bonus, this also fixes the issue of gen_context and the header not being
rebuilt when gen_context.c is touched.

Makefile.am

@@ -86,7 +86,8 @@ gen_%.o: src/gen_%.c
 gen_context$(BUILD_EXEEXT): $(gen_context_OBJECTS)
 	$(CC_FOR_BUILD) $^ -o $@
 
-BUILT_SOURCES = src/ecmult_static_context.h
+$(libsecp256k1_la_OBJECTS): src/ecmult_static_context.h
+$(tests_OBJECTS): src/ecmult_static_context.h
 
 src/ecmult_static_context.h: gen_context
 	./gen_context