package/spirv-tools: needs C++17

Fix the following build failure raised since the addition of the package
in commit 0a01085abeb7d8a097cb68b9f7f4faec7711543c:

CMake Error at CMakeLists.txt:17 (project):
  No CMAKE_CXX_COMPILER could be found.

Fixes:
 - http://autobuild.buildroot.org/results/aff5b968342bf05f036c8e1e557c404060345d30

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: needs C++ for itself, drop inherited comment]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/spirv-tools/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_SPIRV_TOOLS
 	bool "spirv-tools"
-	depends on BR2_INSTALL_LIBSTDCPP # spirv-headers
+	depends on BR2_INSTALL_LIBSTDCPP
+	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_7 # C++17
 	select BR2_PACKAGE_SPIRV_HEADERS
 	help
 	  The SPIR-V Tools project provides an API and commands for
@@ -8,5 +9,6 @@ config BR2_PACKAGE_SPIRV_TOOLS
 
 	  https://github.com/KhronosGroup/SPIRV-Tools
 
-comment "spirv-tools needs a toolchain w/ C++"
-	depends on !BR2_INSTALL_LIBSTDCPP
+comment "spirv-tools needs a toolchain w/ C++, gcc >= 7"
+	depends on !BR2_INSTALL_LIBSTDCPP || \
+		!BR2_TOOLCHAIN_GCC_AT_LEAST_7