Check dl library existence (fix #990)

As FreeBSD includes dl functions in libc, dl library doesn't exist.
(dlfcn.h header file must exist anyway.)

src/base/CMakeLists.txt

@@ -20,6 +20,13 @@ int main() {
 }
 " HAVE_SCHED_YIELD)
 
+check_c_source_compiles("
+#include <dlfcn.h>
+int main() {
+  return 0;
+}
+" HAVE_DLFCN_H)
+
 check_c_source_runs("
 int main() {
   unsigned long value = 0x40302010ul;
@@ -85,5 +92,8 @@ target_link_libraries(base-lib modp_b64)
 if(WIN32)
   target_link_libraries(base-lib dbghelp shlwapi)
 else()
-  target_link_libraries(base-lib dl)
+  find_library(DL_LIBRARY NAMES dl)
+  if(DL_LIBRARY_FOUND)
+    target_link_libraries(base-lib ${DL_LIBRARY})
+  endif()
 endif()

src/base/config.h.cmakein

@@ -1,5 +1,5 @@
 // Aseprite Base Library                                      -*- C++ -*-
-// Copyright (c) 2001-2015 David Capello
+// Copyright (c) 2001-2016 David Capello
 //
 // This file is released under the terms of the MIT license.
 // Read LICENSE.txt for more information.
@@ -10,6 +10,7 @@
 
 #cmakedefine HAVE_STDINT_H
 #cmakedefine HAVE_SCHED_YIELD
+#cmakedefine HAVE_DLFCN_H
 
 #cmakedefine ASEPRITE_LITTLE_ENDIAN
 #cmakedefine ASEPRITE_BIG_ENDIAN

src/base/dll_unix.h

@@ -5,7 +5,12 @@
 // Read LICENSE.txt for more information.
 
 #include "base/string.h"
-#include <dlfcn.h>
+
+#ifdef HAVE_DLFCN_H
+  #include <dlfcn.h>
+#else
+  #error dlfcn.h is needed or include a file that defines dlopen/dlclose
+#endif
 
 namespace base {