diff --git a/builder/package.centos-6 b/builder/package.centos-6
index 8ad4c91..eb89fae 100644
--- a/builder/package.centos-6
+++ b/builder/package.centos-6
@@ -20,7 +20,9 @@ export PATH=/usr/local/rvm/rubies/ruby-2.6.3/bin/:/.gems/bin:${PATH}
 export GEM_HOME=/.gems
 export GEM_PATH=/.gems
 
-# Additional config for c++11 support
+# Additional config for C++11 support in R 3.6.
+# R 3.6 no longer sets CXX11 flags without a full C++11 compiler present, but
+# the default gcc 4.4.7 still partially supports C++11 through -std=gnu++0x.
 sed -i 's/^CXX11\ =\ .*/CXX11\ =\ g++/g' /opt/R/${R_VERSION}/lib/R/etc/Makeconf
 sed -i 's/^CXX11FLAGS\ =\ .*/CXX11FLAGS\ =\ -g\ -O2\ \$\(LTO\)/g' /opt/R/${R_VERSION}/lib/R/etc/Makeconf
 sed -i 's/^CXX11PICFLAGS\ =\ .*/CXX11PICFLAGS\ =\ -fpic/g' /opt/R/${R_VERSION}/lib/R/etc/Makeconf