Replace os.makedirs() with 'mkdir -p'

Resolves issue with concurrent workers creating directories. E.g.,

File "PKGTOOLS/cmsBuild", line 2805, in buildPackage
makedirs (pkg.builddir)
File "/usr/lib64/python2.4/os.py", line 156, in makedirs
makedirs(head, mode)
File "/usr/lib64/python2.4/os.py", line 156, in makedirs
makedirs(head, mode)
File "/usr/lib64/python2.4/os.py", line 159, in makedirs
mkdir(name, mode)
OSError: [Errno 17] File exists: '/<snip>/a/BUILD/slc5_amd64_gcc472/external'

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Signed-off-by: Giulio Eulisse <giulio.eulisse@gmail.com>
Merge pull request #12 from cms-sw/fix-mkdirs-race-condition

Replace os.makedirs() with 'mkdir -p'