BUILD: Disable -Wunused-const-variable

This one is enabled by -Wall in Clang, but not in gcc (at least not
for C++). The warning doesn't make sense for us, especially since we
sometimes leave in unused static const values for documentation
purposes, so let's disable it.

CMakeLists.txt

@@ -108,6 +108,9 @@ if (NOT "${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
 
   # We do not yet mark any of our overrides. Let's disable this for now
   set_check_compiler_flag_cxx("-Winconsistent-missing-override" "-Wno-inconsistent-missing-override")
+
+  # Disable warnings about unused static const variables
+  set_check_compiler_flag_cxx("-Wunused-const-variable" "-Wno-unused-const-variable")
 endif()
 
 # platform specific warning settings

configure.ac

@@ -140,6 +140,9 @@ AX_CHECK_COMPILER_FLAGS_VAR([C++], [WARN], [-Wundefined-var-template], [-Wno-und
 dnl We do not yet mark any of our overrides. Let's disable this for now
 AX_CHECK_COMPILER_FLAGS_VAR([C++], [WARN], [-Winconsistent-missing-override], [-Wno-inconsistent-missing-override])
 
+dnl Disable warnings about unused static const variables
+AX_CHECK_COMPILER_FLAGS_VAR([C++], [WARN], [-Wunused-const-variable], [-Wno-unused-const-variable])
+
 AC_SUBST(WARN)
 
 dnl C++ dialect