Add define that avoids using std::unary_function when using c++17 or …

…newer standard.

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

@@ -590,10 +590,10 @@ def determine_b2_options(self, spec, options):
         cxxflags = []
 
         # Deal with C++ standard.
+        cxxstd = spec.variants["cxxstd"].value
         if spec.satisfies("@1.66:"):
             options.append("cxxstd={0}".format(spec.variants["cxxstd"].value))
         else:  # Add to cxxflags for older Boost.
-            cxxstd = spec.variants["cxxstd"].value
             flag = getattr(self.compiler, "cxx{0}_flag".format(cxxstd))
             if flag:
                 cxxflags.append(flag)
@@ -607,6 +607,12 @@ def determine_b2_options(self, spec, options):
         if not spec.satisfies("@:1.70 %intel"):
             cxxflags.append("-DBOOST_PARAMETER_DISABLE_PERFECT_FORWARDING")
 
+        # std::unary_function was removed in C++17 so need to set the following
+        # flag for building boost
+        if (cxxstd == "17") or (cxxstd == "2a") or (cxxstd == "20") or
+           (cxxstd == "23") or (cxxstd == "26"):
+            cxxflags.append("-DBOOST_NO_CXX98_FUNCTION_BASE")
+
         # clang is not officially supported for pre-compiled headers
         # and at least in clang 3.9 still fails to build
         #   https://www.boost.org/build/doc/html/bbv2/reference/precompiled_headers.html