CMake: Improve finding Iconv

Continue when Iconv can not be found.
Silence warnings when testing if second argument for iconv() is const.
Use correct include dir.

build/cmake/modules/FindIconv.cmake

@@ -25,6 +25,7 @@ ENDIF(ICONV_INCLUDE_DIR AND ICONV_LIBRARIES)
 
 set(CMAKE_REQUIRED_INCLUDES ${ICONV_INCLUDE_DIR})
 set(CMAKE_REQUIRED_LIBRARIES ${ICONV_LIBRARIES})
+set(CMAKE_REQUIRED_QUIET ON)
 IF(ICONV_FOUND)
   check_cxx_source_compiles("
   #include <iconv.h>
@@ -41,6 +42,7 @@ IF(ICONV_FOUND)
 ENDIF(ICONV_FOUND)
 set(CMAKE_REQUIRED_INCLUDES)
 set(CMAKE_REQUIRED_LIBRARIES)
+set(CMAKE_REQUIRED_QUIET)
 
 IF(ICONV_FOUND)
   IF(NOT ICONV_FIND_QUIETLY)