boost: context needs C++11

Without std=c++11, mutex is not always correctly detected with gcc 4.7, 4.8 or even 5.x. As a result, boost_thread is wrongly selected by boost_context.

mutex is available on gcc 4.7.x, the issue is that boost does not correctly detect it because -std=c++11 was missing.
Keep thread select for gcc version lower or equal to 4.6. mutex is available since gcc 4.4 (with std=c++0x) but common buildroot practice for C++11 packages is to have a dependency on gcc 4.7, 4.8 or 4.9 depending on the requested C++11 features.

Fixes:
- http://autobuild.buildroot.net/results/fb046c04fe18bec973d120e4ab33971f32ba5769
- http://autobuild.buildroot.net/results/99f1a255b78a973faeb8bf3b94d78efc54426a8b

---
Changes v2 -> v3 (after review of Thomas Petazzoni):
- Merge both patches
- Update commit message

Changes v1 -> v2 (after review of Thomas Petazzoni):
- Update first patch to keep selection of boost-thread but lower gcc version from 4.8 to 4.7
- Update second patch to add -std=c++11 only if gcc >= 4.7

 package/boost/Config.in | 2 +-
 package/boost/boost.mk  | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

index 965d198ebf..627cf2f356 100644
@@ -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_8
+	select BR2_PACKAGE_BOOST_THREAD if !BR2_TOOLCHAIN_GCC_AT_LEAST_4_7
 	help
 	  C++11 context switching library.
 

index 14f6162003..f4a44fa123 100644
@@ -61,6 +61,11 @@ 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

2.14.1

Hello,

Applied to master, thanks.

Thomas

Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com