makepkg: disable extglob when sourcing BUILDSCRIPT
PKGBUILDs are advertised as being pure bash so it would be expected
that the default bash options are in effect when sourcing it.
This inadvertantly "fixes" FS#27780 where enabling extglob causes
the bash parser to error on non-valid bash function names like
package_libxml++(). Note that these function names are unsupported
in bash and could break again even with this "fix" in future bash