Fix configure error on Linux with DCMTK_ENABLE_CXX11 enabled

Configuring on Linux with DCMTK_ENABLE_CXX11 enabled generates an error in the
configuration step:

    CMake Error at CMake/GenerateDCMTKConfigure.cmake:1252 (MESSAGE):
      ...
      /path/to/dcmtk/config/tests/../math.cc:88:12: error:
      '::isnan' has not been declared

           return ::isnan(f);

One way around this is to ensure that the preferred std::isnan() function is
used instead. This commit fixes the following error in the check for that
function by specifying the double overload:

    FAILED: CMakeFiles/cmTC_62c88.dir/CheckSymbolExists.cxx.o
    /path/to/dcmtk-build/CMakeTmp/CheckSymbolExists.cxx:29:11: error: statement cannot resolve address of overloaded function
     std::isnan;return 0;

CMake/GenerateDCMTKConfigure.cmake

@@ -571,8 +571,8 @@ ENDIF(WIN32 AND NOT CYGWIN)
   CHECK_FUNCTIONWITHHEADER_EXISTS(isinf "${HEADERS}" HAVE_PROTOTYPE_ISINF)
   CHECK_FUNCTIONWITHHEADER_EXISTS(isnan "${HEADERS}" HAVE_PROTOTYPE_ISNAN)
   CHECK_FUNCTIONWITHHEADER_EXISTS(finite "${HEADERS}" HAVE_PROTOTYPE_FINITE)
-  CHECK_FUNCTIONWITHHEADER_EXISTS(std::isinf "${HEADERS}" HAVE_PROTOTYPE_STD__ISINF)
-  CHECK_FUNCTIONWITHHEADER_EXISTS(std::isnan "${HEADERS}" HAVE_PROTOTYPE_STD__ISNAN)
+  CHECK_FUNCTIONWITHHEADER_EXISTS("std::isinf(0.)" "${HEADERS}" HAVE_PROTOTYPE_STD__ISINF)
+  CHECK_FUNCTIONWITHHEADER_EXISTS("std::isnan(0.)" "${HEADERS}" HAVE_PROTOTYPE_STD__ISNAN)
   CHECK_FUNCTIONWITHHEADER_EXISTS(std::finite "${HEADERS}" HAVE_PROTOTYPE_STD__FINITE)
   CHECK_FUNCTIONWITHHEADER_EXISTS(flock "${HEADERS}" HAVE_PROTOTYPE_FLOCK)
   CHECK_FUNCTIONWITHHEADER_EXISTS(gethostbyname "${HEADERS}" HAVE_PROTOTYPE_GETHOSTBYNAME)