gnu: apr: Build sequentially.

* gnu/packages/apr.scm (apr)[arguments]: New field.

gnu/packages/apr.scm

@@ -40,6 +40,11 @@
               (list (search-patch "apr-skip-getservbyname-test.patch")))
              (patch-flags '("-p0"))))
     (build-system gnu-build-system)
+    (arguments
+     ;; Sometimes we end up with two processes concurrently trying to make
+     ;; 'libmod_test.la': <http://hydra.gnu.org/build/60266/nixlog/2/raw>.
+     ;; Thus, build sequentially.
+     '(#:parallel-build? #f))
     (inputs `(("perl" ,perl)
               ("libtool" ,libtool)))
     (home-page "http://apr.apache.org/")