ticket:11003 Delete cmutex.h; change files using it to from plain C to C++.

git-svn-id: http://source.icu-project.org/repos/icu/icu@36050 251d0590-4201-4cf1-90de-194747b24ca1
ticket:11003 Clean up gcc compiler warnings introduced with previous commit for this ticket.

git-svn-id: http://source.icu-project.org/repos/icu/icu@36051 251d0590-4201-4cf1-90de-194747b24ca1