Fix permissions error installing to non-owned directory

When installing to a directory which is not owned by the installing
user, even when the user has write permission for the directory, the
installation can fail with an error similar to the following:

Installing libblis-0.1.4-7-sandybridge.a into /usr/local/lib/
install: cannot change permissions of ‘/usr/local/lib’: Operation not permitted
Makefile:658: recipe for target '/usr/local/lib/libblis-0.1.4-7-sandybridge.a' failed
make: *** [/usr/local/lib/libblis-0.1.4-7-sandybridge.a] Error 1

In the example case, the error occurred because the user attempted to
install to /usr/local and /usr/local/lib is owned by root with mode 2755
which the Makefile unsuccessfully attempted to change to 0755.

Given that installing to /usr/local is likely to be quite common and the
ownership/permissions are the default for Debian and Debian-derived
Linux distributions (perhaps others as well), this commit attempts to
support that use case by using mkdir rather than install to create the
directory (which is the same approach as Automake).

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Merge pull request #19 from kevinoid/fix-install-perms-error

Fix permissions error installing to non-owned directory