cmake: Try to fix FindArgp module

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

tests/CMakeLists.txt

@@ -31,10 +31,12 @@ endif (LIBSSH_THREADS)
 
 # create test library
 add_library(${TORTURE_LIBRARY} STATIC cmdline.c torture.c)
-target_link_libraries(${TORTURE_LIBRARY}
-    ${TORTURE_LINK_LIBRARIES}
-    ${ARGP_LIBRARIES}
-)
+if (ARGP_LIBRARY)
+    target_link_libraries(${TORTURE_LIBRARY}
+        ${TORTURE_LINK_LIBRARIES}
+        ${ARGP_LIBRARY}
+    )
+endif()
 
 set(TEST_TARGET_LIBRARIES
     ${TORTURE_LIBRARY}