Include <boost/config.h> in the build/* detectors

Fixes: https://github.com/boostorg/stacktrace/issues/229

build/has_addr2line.cpp

@@ -6,6 +6,7 @@
 
 #include <cstdlib>
 
+// pick up "user.hpp" with the required defines such as BOOST_STACKTRACE_ADDR2LINE_LOCATION
 #include <boost/config.hpp>
 #include <unwind.h>
 #include <sys/types.h>

build/has_backtrace.cpp

@@ -4,6 +4,9 @@
 // accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
+// pick up "user.hpp" with the required defines such as BOOST_STACKTRACE_BACKTRACE_INCLUDE_FILE
+#include <boost/config.hpp>
+
 #ifdef BOOST_STACKTRACE_BACKTRACE_INCLUDE_FILE
 #   include BOOST_STACKTRACE_BACKTRACE_INCLUDE_FILE
 #else