Fix boost (>= 1.66) on gcc 4.9.3

[muffgaga]

When trying to use boost 1.66 on gcc 4.9.3 (not sure about 4.9.4; we use gcc 4.9.3 as it is the last "known good" version for gccxml) we encountered a strange bug related to gcc's handling of __has_include:
The macro is defined but not implemented correctly.
Our workaround was to conditionally undefine the macro when encountering gcc 4.9.x.

diff -rpu boost_1_66_0_old/boost/config/stdlib/libstdcpp3.hpp boost_1_66_0/boost/config/stdlib/libstdcpp3.hpp
--- boost_1_66_0_old/boost/config/stdlib/libstdcpp3.hpp	2017-12-14 00:56:42.000000000 +0100
+++ boost_1_66_0/boost/config/stdlib/libstdcpp3.hpp	2018-04-25 16:42:42.627694409 +0200
@@ -301,6 +301,11 @@ extern "C" char *gets (char *__s);
 #  define BOOST_NO_CXX17_STD_APPLY
 #endif
 
+#if defined(__GNUC__) && (__GNUC__ == 4) && (__GNUC_MINOR__ == 9) && defined(__has_include)
+// gcc 4.9.x defines but does not implement it
+#undef __has_include
+#endif
+
 #if defined(__has_include)
 #if !__has_include(<shared_mutex>)
 #  define BOOST_NO_CXX14_HDR_SHARED_MUTEX

[jzmaddock]

That's nasty, but the fix looks almost as bad.... not sure what to do about this one.