boost: context needs thread if gcc < 6

Commit f3a483a81be43b0ff674400e11900605969f8f1c added -std=c++11 to
boost build if context is selected and gcc is at least 4.7 however it
has the side effect that cc-tool fails to find boost_system with the
following error if context and system is enabled with a gcc greater
than 6:

configure:16312: /home/dawncrow/buildroot-test/scripts/instance-0/output/host/bin/x86_64-linux-g++ -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -I/home/dawncrow/buildroot-test/scripts/instance-0/output/host/x86_64-buildroot-linux-musl/sysroot/usr/include  -L/home/dawncrow/buildroot-test/scripts/instance-0/output/host/x86_64-buildroot-linux-musl/sysroot/usr/lib conftest.o -lboost_system -latomic >&5
conftest.o: In function `boost::system::error_category::std_category::equivalent(int, std::error_condition const&) const':
conftest.cpp:(.text._ZNK5boost6system14error_category12std_category10equivalentEiRKSt15error_condition[_ZNK5boost6system14error_category12std_category10equivalentEiRKSt15error_condition]+0x37): undefined reference to `boost::system::detail::generic_category_instance'

This error is related to the fact that since boost 1.68 and commit
https://github.com/boostorg/system/commit/7b6dcf6ac668e24f11ec5dbc68d1fa0248d964df,
boost system headers are now C++14 "by default" with gcc above 6:
- https://github.com/boostorg/system/issues/24
- https://github.com/boostorg/system/issues/26
- https://lists.boost.org/Archives/boost/2018/08/242770.php

So when building with gcc > 6, cc-tool thinks that boost has
generic_category_instance but because boost was compiled with
std=c++11, this function will not be in the library causing a link error

Instead of "hacking" even more boost, just remove -std=c++11 from
boost.mk and select BOOST_THREAD with gcc lower than 6

Fixes:
 - http://autobuild.buildroot.org/results/fc8f8a64751c751b2b66301967cc008509bbaa70

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Reviewed-by: Ferdinand van Aartsen <ferdinand@ombud.nl>
Tested-by: Ferdinand van Aartsen <ferdinand@ombud.nl>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/boost/Config.in

@@ -83,7 +83,7 @@ config BR2_PACKAGE_BOOST_CONTEXT
 	bool "boost-context"
 	depends on BR2_PACKAGE_BOOST_CONTEXT_ARCH_SUPPORTS
 	depends on !BR2_TOOLCHAIN_HAS_GCC_BUG_64735
-	select BR2_PACKAGE_BOOST_THREAD if !BR2_TOOLCHAIN_GCC_AT_LEAST_4_7
+	select BR2_PACKAGE_BOOST_THREAD if !BR2_TOOLCHAIN_GCC_AT_LEAST_6
 	help
 	  C++11 context switching library.
 

package/boost/boost.mk

@@ -61,11 +61,6 @@ else
 BOOST_FLAGS += --without-icu
 endif
 
-# boost-context needs C++11
-ifeq ($(BR2_PACKAGE_BOOST_CONTEXT)$(BR2_TOOLCHAIN_GCC_AT_LEAST_4_7),yy)
-BOOST_TARGET_CXXFLAGS += -std=c++11
-endif
-
 ifeq ($(BR2_PACKAGE_BOOST_IOSTREAMS),y)
 BOOST_DEPENDENCIES += bzip2 zlib
 endif