--- a/build/moz.configure/toolchain.configure
+++ b/build/moz.configure/toolchain.configure
@@ -143,16 +143,20 @@ def toolchain_prefix(value, target, host
         return '%s-' % target.toolchain
 
 set_config('TOOLCHAIN_PREFIX', toolchain_prefix)
 add_old_configure_assignment('TOOLCHAIN_PREFIX', toolchain_prefix)
 
 
 # Compilers
 # ==============================================================
+option('--enable-legacy-toolchain', env='MOZ_ENABLE_LEGACY_TOOLCHAIN',
+       help='Allow building with legacy toolchains')
+
+@depends('--enable-legacy-toolchain')
 @imports('os')
 @imports('subprocess')
 @imports(_from='mozbuild.configure.util', _import='LineIO')
 @imports(_from='mozbuild.shellutil', _import='quote')
 @imports(_from='tempfile', _import='mkstemp')
 @imports(_from='textwrap', _import='dedent')
 def check_compiler(compiler, language):
     check = dedent('''\

--- a/old-configure.in
+++ b/old-configure.in
@@ -359,16 +359,19 @@ case "$target" in
             AC_MSG_ERROR([The major versions of \$CC and \$CXX do not match.])
         fi
 
         AC_DEFINE(_CRT_SECURE_NO_WARNINGS)
         AC_DEFINE(_CRT_NONSTDC_NO_WARNINGS)
         AC_DEFINE(_USE_MATH_DEFINES) # Otherwise MSVC's math.h doesn't #define M_PI.
 
         if test "$_CC_MAJOR_VERSION" = "18" -a "$_CC_BUILD_VERSION" -ge "30723"; then
+            if [ -z "$MOZ_ENABLE_LEGACY_TOOLCHAIN" ]; then
+                AC_MSG_ERROR([Visual Studio 2015 is required to build; override this warning by adding --enable-legacy-toolchain])
+            fi
             _CC_SUITE=12
             MSVS_VERSION=2013
             MSVC_C_RUNTIME_DLL=msvcr120.dll
             MSVC_CXX_RUNTIME_DLL=msvcp120.dll
         elif test "$_CC_MAJOR_VERSION" = "19" -a "$_CC_BUILD_VERSION" -ge "23506"; then
             _CC_SUITE=14
             MSVS_VERSION=2015
             MSVC_C_RUNTIME_DLL=vcruntime140.dll