libsigc: bump gcc version requirement to 4.8

It now uses C++11 features that aren't available in 4.7. Fixes:
http://autobuild.buildroot.net/results/405/40563404e2612590a6ad19cbb39df14074fe160e/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libsigc/Config.in

@@ -1,6 +1,6 @@
 config BR2_PACKAGE_LIBSIGC
 	bool "libsigc++"
-	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 # C++11
+	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_8 # C++11
 	depends on BR2_INSTALL_LIBSTDCPP
 	help
 	  libsigc++ implements a typesafe callback system for standard C++.
@@ -10,5 +10,5 @@ config BR2_PACKAGE_LIBSIGC
 
 	  http://libsigc.sourceforge.net/
 
-comment "libsigc++ needs a toolchain w/ C++, gcc >= 4.7"
-	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_GCC_AT_LEAST_4_7
+comment "libsigc++ needs a toolchain w/ C++, gcc >= 4.8"
+	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_GCC_AT_LEAST_4_8