biology/lamarc: Fix build with Clang 6

src/tools/mathx.cpp:781:12: error: no matching function for call to 'make_pair'
    return std::make_pair<DoubleVec1d, DoubleVec2d>(eigvals, eigvecs);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/v1/utility:639:1: note: candidate function not viable: no known conversion from 'DoubleVec1d' (aka 'vector<double>') to 'std::__1::vector<double, std::__1::allocator<double> > &&' for 1st argument
make_pair(_T1&& __t1, _T2&& __t2)
^

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/lamarc-2.1.8_1,1.log

biology/lamarc/Makefile

@@ -14,6 +14,8 @@ COMMENT=	Package of programs for computing population parameters
 
 LICENSE=	APACHE20
 
+USE_CXXSTD=	c++98
+
 WRKSRC=		${WRKDIR}/${DISTNAME:S,${DISTVERSIONSUFFIX},,}
 
 GNU_CONFIGURE=	yes