apr: remove the use of ${SHELL} to avoid bash/dash confliction

While multible hosts sharing a common sstate cache, the fist host using
bash as default shell and build apr, the second host using dash as
default shell and build apr-util, there was a failure in apr-util:
...
| /bin/sh: 0: Can't open i586-poky-linux-libtool
| make[1]: *** [dbm/apr_dbm_gdbm.lo] Error 127
| make[1]: *** Waiting for unfinished jobs....
| make[1]: Leaving directory
`tmp/work/i586-poky-linux/apr-util/1.5.2-r0/apr-util-1.5.2'
...

The quick way to reproduce the defect in Ubuntu 1204:
1. Create a new build
2. sudo dpkg-reconfigure dash <set bash as /bin/sh>
3. bitbake apr
4. sudo dpkg-reconfigure dash <set dash as /bin/sh>
5. bitbake apr-util

Remove the use of $(SHELL) in the apr could avoid this issue.

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>