Make iconv test more strict

Since some systems (FreeBSD) use macro magic to chose between iconv
library (GNU vs. internal) versions, including iconv.h is crucial here.
This fixed build on FreeBSD 10+ with libiconv installed from ports.

Author: AMDmi3

cmake/FindIconv.cmake

@@ -11,10 +11,12 @@ FIND_PATH(ICONV_INCLUDE_DIR NAMES iconv.h)
 
 # Check if library is needed
 IF(ICONV_INCLUDE_DIR)
-	INCLUDE(CheckFunctionExists)
-
 	SET(CMAKE_REQUIRED_INCLUDES ${ICONV_INCLUDE_DIR})
-	CHECK_FUNCTION_EXISTS(iconv ICONV_LIBRARY_NOT_NEEDED)
+	TRY_COMPILE(ICONV_LIBRARY_NOT_NEEDED
+		${CMAKE_BINARY_DIR}
+		${CMAKE_MODULE_PATH}/iconv_test.c
+		CMAKE_FLAGS "-DINCLUDE_DIRECTORIES:STRING=${ICONV_INCLUDE_DIR}"
+	)
 
 	IF(NOT ICONV_LIBRARY_NOT_NEEDED)
 		# Locate library

cmake/iconv_test.c

@@ -0,0 +1,6 @@
+#include <iconv.h>
+
+int main() {
+	iconv_open("", "");
+	return 0;
+}