create cblas.pc link, too, not only blas and lapack

This is needed to make numpy Sage's installer happy,
otherwise pkgconfig might error out, see trac #27870
for discussion.

build/pkgs/openblas/spkg-configure.m4

@@ -3,6 +3,7 @@ SAGE_SPKG_CONFIGURE([openblas], [
     PKG_CHECK_VAR([OPENBLASPCDIR], [openblas], [pcfiledir], [
        AC_CONFIG_LINKS([
          $SAGE_LOCAL/lib/pkgconfig/blas.pc:$OPENBLASPCDIR/openblas.pc
+         $SAGE_LOCAL/lib/pkgconfig/cblas.pc:$OPENBLASPCDIR/openblas.pc
          $SAGE_LOCAL/lib/pkgconfig/lapack.pc:$OPENBLASPCDIR/openblas.pc])
        ], [
        AC_MSG_WARN([Unable to locate the directory of openblas.pc. This should not happen!])