Fix boost@:1.56.0%gcc@:5 (#4295)

I need boost@1.54.0 for bcl2fastq2.  I tested bcl2fastq2 using the
system compiler (gcc@4.8.5).  My full build uses a spack-build
gcc@5.4.0.

boost@1.54.0 won't build.  I found the answer
[here](https://github.com/hashdist/hashstack/issues/802) with the
details in [this upstream/boost
ticket](https://svn.boost.org/trac/boost/ticket/10125).

I've confirmed that these combo's build:

boost version| compiler
------------ | ------------------
boost@1.54.0 | gcc@4.8.5 (system)
boost@1.54.0 | gcc@5.4.0 (Spack)
boost@1.64.0 | gcc@5.5.0 (system)

var/spack/repos/builtin/packages/boost/package.py

@@ -148,6 +148,8 @@ class Boost(Package):
     patch('xl_1_62_0_le.patch', when='@1.62.0%xl_r')
     patch('xl_1_62_0_le.patch', when='@1.62.0%xl')
 
+    patch('call_once_variadic.patch', when='@:1.56.0%gcc@5:')
+
     def url_for_version(self, version):
         url = "http://downloads.sourceforge.net/project/boost/boost/{0}/boost_{1}.tar.bz2"
         return url.format(version.dotted, version.underscored)