Bug 1481097 - mfbt: Assert we're compiling with gcc 6.1 or later. r=glandium

Firefox currently requires gcc 6.1 or later (as of bug 1444274).

MozReview-Commit-ID: CaJwpUgntxn

--HG--
extra : rebase_source : 302d8aa57696e2e96962c67e3497f76e882a472f
extra : source : 2a53a830f79424b2c41c8bdc4816ec24fa943ecc

mfbt/Compiler.h

@@ -26,8 +26,8 @@
 #  define MOZ_GCC_VERSION_AT_MOST(major, minor, patchlevel)           \
      ((__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) \
       <= ((major) * 10000 + (minor) * 100 + (patchlevel)))
-#  if !MOZ_GCC_VERSION_AT_LEAST(4, 9, 0)
-#    error "mfbt (and Gecko) require at least gcc 4.9 to build."
+#  if !MOZ_GCC_VERSION_AT_LEAST(6, 1, 0)
+#    error "mfbt (and Gecko) require at least gcc 6.1 to build."
 #  endif
 
 #elif defined(_MSC_VER)