package/flann: needs C++11 support

Fixes:

  - http://autobuild.buildroot.net/results/c92a3df1bc73e00e1e4493500bfb7277cf5064ee

  .../build/flann-1.9.2/src/cpp/flann/util/lsh_table.h:367:5: error: 'random_device' is not a member of 'std'
     std::random_device rd;
     ^

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/flann/Config.in

@@ -2,6 +2,7 @@ config BR2_PACKAGE_FLANN
 	bool "flann"
 	depends on BR2_INSTALL_LIBSTDCPP
 	depends on !BR2_STATIC_LIBS
+	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 # C++11
 	select BR2_PACKAGE_LZ4
 	help
 	  FLANN is a library for performing fast approximate nearest
@@ -22,5 +23,6 @@ config BR2_PACKAGE_FLANN_EXAMPLES
 
 endif
 
-comment "flann needs a toolchain w/ C++, dynamic library"
-	depends on !BR2_INSTALL_LIBSTDCPP || BR2_STATIC_LIBS
+comment "flann needs a toolchain w/ C++, dynamic library, gcc >= 4.7"
+	depends on !BR2_INSTALL_LIBSTDCPP || BR2_STATIC_LIBS || \
+		!BR2_TOOLCHAIN_GCC_AT_LEAST_4_7

package/flann/flann.mk

@@ -18,7 +18,7 @@ FLANN_CONF_OPTS = \
 	-DUSE_OPENMP=$(if $(BR2_TOOLCHAIN_HAS_OPENMP),ON,OFF) \
 	-DCMAKE_DISABLE_FIND_PACKAGE_HDF5=TRUE
 
-FLANN_CXXFLAGS = $(TARGET_CXXFLAGS)
+FLANN_CXXFLAGS = $(TARGET_CXXFLAGS) -std=c++11
 
 ifeq ($(BR2_TOOLCHAIN_HAS_GCC_BUG_68485),y)
 FLANN_CXXFLAGS += -O0