backport: regex.c: Declare wcs functions only if compiling with MBS_SUPPORT.

backport: regex.c: Declare wcs functions only if compiling with MBS_SUPPORT.

Merge from glibc:
        * regex.c: Declare wcs functions only if compiling with
        MBS_SUPPORT.
        Don't use #elif for traditional C.

        * regex.c: Revamp memory allocation for WCHAR functions to
        not use too much stack.

From-SVN: r44471