Fixes compile for m68k.

From upstream double-conversion commit da111796231:
https://github.com/google/double-conversion/blob/master/double-conversion/utils.h

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>

diff -Nur poco-poco-1.7.2-release.orig/Foundation/src/utils.h poco-poco-1.7.2-release/Foundation/src/utils.h
--- poco-poco-1.7.2-release.orig/Foundation/src/utils.h	2016-03-19 08:19:35.000000000 +0100
+++ poco-poco-1.7.2-release/Foundation/src/utils.h	2016-08-11 02:55:02.827238119 +0200
@@ -63,6 +63,8 @@
     defined(__AARCH64EL__) || \
     defined(nios2) || defined(__nios2) || defined(__nios2__)
 #define DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS 1
+#elif defined(__mc68000__)
+#undef DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS
 #elif defined(_M_IX86) || defined(__i386__) || defined(__i386)
 #if defined(_WIN32)
 // Windows uses a 64bit wide floating point stack.