gnu: Use `mirror://apache' where applicable.

* gnu/packages/libapr.scm (libapr, libaprutil): Use `mirror://apache'.
* gnu/packages/subversion.scm (subversion): Likewise.

gnu: Add libaprutil.

* gnu/packages/libapr: new variable.

gnu: Add libapr.

* gnu/packages/libapr.scm: New file.
* gnu/packages/patches/libapr-skip-getservbyname-test.patch: New file
* Makefile.am: Add them