add direct source dependency to each library

Fix library generation with some CMake generators by including a direct
source file dependency for each library.

tls/CMakeLists.txt

@@ -48,7 +48,7 @@ target_include_directories(tls_obj
 		../include)
 
 add_library(tls $<TARGET_OBJECTS:tls_obj> $<TARGET_OBJECTS:ssl_obj>
-	$<TARGET_OBJECTS:crypto_obj>)
+	$<TARGET_OBJECTS:crypto_obj> empty.c)
 
 export_symbol(tls ${CMAKE_CURRENT_BINARY_DIR}/tls.sym)
 target_link_libraries(tls ${PLATFORM_LIBS})

tls/Makefile.am

@@ -8,6 +8,7 @@ lib_LTLIBRARIES = libtls.la
 EXTRA_DIST = VERSION
 EXTRA_DIST += CMakeLists.txt
 EXTRA_DIST += tls.sym
+EXTRA_DIST += empty.c
 
 CLEANFILES = libtls_la_objects.mk