cmake: Don't try to use readline if you don't also have curses.

Fixes #17.

CMakeLists.txt

@@ -209,13 +209,15 @@ if(PHYSFS_BUILD_TEST)
     find_path(HISTORY_H readline/history.h)
     if(READLINE_H AND HISTORY_H)
         find_library(CURSES_LIBRARY NAMES curses ncurses)
-        set(CMAKE_REQUIRED_LIBRARIES ${CURSES_LIBRARY})
-        find_library(READLINE_LIBRARY readline)
-        if(READLINE_LIBRARY)
-            set(HAVE_SYSTEM_READLINE TRUE)
-            set(TEST_PHYSFS_LIBS ${TEST_PHYSFS_LIBS} ${READLINE_LIBRARY} ${CURSES_LIBRARY})
-            include_directories(SYSTEM ${READLINE_H} ${HISTORY_H})
-            add_definitions(-DPHYSFS_HAVE_READLINE=1)
+        if(CURSES_LIBRARY)
+            set(CMAKE_REQUIRED_LIBRARIES ${CURSES_LIBRARY})
+            find_library(READLINE_LIBRARY readline)
+            if(READLINE_LIBRARY)
+                set(HAVE_SYSTEM_READLINE TRUE)
+                set(TEST_PHYSFS_LIBS ${TEST_PHYSFS_LIBS} ${READLINE_LIBRARY} ${CURSES_LIBRARY})
+                include_directories(SYSTEM ${READLINE_H} ${HISTORY_H})
+                add_definitions(-DPHYSFS_HAVE_READLINE=1)
+            endif()
         endif()
     endif()
     add_executable(test_physfs test/test_physfs.c)