x11-toolkits/gtkmathview: Fix build with Clang 6

View.cc:294:10: error: no viable conversion from returned value of type 'bool' to function return type 'SmartPtr<Element>'

return false;
       ^~~~~

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/gtkmathview-0.8.0_9.log

cmex10.cc:27:3: error: constant expression evaluates to 4205933842 which cannot be

    narrowed to type 'int' [-Wc++11-narrowing]
0xfab17512,
^~~~~~~~~~

SVG_RenderingContext.cc:67:25: error: invalid suffix on literal; C++11 requires a

    space between literal and identifier [-Wreserved-user-defined-literal]
metadata("Created by "PACKAGE" version "VERSION);
                      ^