Fix FTBFS due to incomplete declaration of main function in CMake tests.

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>

ConfigureChecks.cmake

@@ -41,7 +41,7 @@ endif()
 check_cxx_source_compiles("
   #include <signal.h>
   #include <${SWORD_INCLUDE_DIRS}/sysdata.h>
-  main() {}
+  int main() {}
   "
   SWORD_WITHOUT_SANE_USERSPACE_TYPES
 )
@@ -50,7 +50,7 @@ if( NOT SWORD_WITHOUT_SANE_USERSPACE_TYPES )
     #define __SANE_USERSPACE_TYPES__
     #include <signal.h>
     #include <${SWORD_INCLUDE_DIRS}/sysdata.h>
-    main() {}
+    int main() {}
     "
     SWORD_WITH_SANE_USERSPACE_TYPES
   )