Change definition of unsetenv to follow POSIX.1-2001

Change definition of unsetenv to follow POSIX.1-2001

Prior to glibc 2.2.2, unsetenv() was prototyped as returning void, as was
this version. However, check's unit tests expect unsetenv to return
an integer. On systems where the override version of unsetenv is used,
check would not compile.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@643 64e312b2-a51f-0410-8e61-82d0ca0eb02a