cmake: Check for Argp also on Linux to fix alpine build

This adjusts also usage of ARGP_LIBRARY to use ARGP_LIBRARIES which is defined
by the FindArgp module, unlike the former one in case it is provided by libc
directly.

Fixes: #167

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Norbert Pocs <npocs@redhat.com>

tests/CMakeLists.txt

@@ -73,9 +73,9 @@ if (CLIENT_TESTING)
     )
 endif ()
 
-if (ARGP_LIBRARY)
+if (ARGP_LIBRARIES)
     target_link_libraries(${TORTURE_LIBRARY}
-        ${ARGP_LIBRARY}
+        ${ARGP_LIBRARIES}
     )
 endif()
 

tests/server/test_server/CMakeLists.txt

@@ -30,7 +30,7 @@ if (UNIX AND NOT WIN32)
     target_link_libraries(test_server
                           testserver
                           ssh::ssh
-                          ${ARGP_LIBRARY}
+                          ${ARGP_LIBRARIES}
                           util)
 endif ()