diff --git a/TestScripts/setenv-ios.sh b/TestScripts/setenv-ios.sh
index 532e2df3..43b69d2a 100755
--- a/TestScripts/setenv-ios.sh
+++ b/TestScripts/setenv-ios.sh
@@ -12,6 +12,12 @@
 # licensed under the Boost Software License 1.0, while the individual files
 # in the compilation are all public domain.
 #
+# cpp is set to Apple's cpp. Actually, cpp is merely on-path so Apple's cpp
+# is used. But Apple's cpp is sufficiently different from GNU's cpp and causes
+# Autotools a lot of trouble because Autotools tests are predicated on GNU cpp.
+# If your Autotools project results in "configure:6560: error: C preprocessor
+# cpp fails sanity check", then file a bug report with Autotools.
+#
 # See http://www.cryptopp.com/wiki/iOS_(Command_Line) for more details
 #############################################################################
 
diff --git a/TestScripts/setenv-macos.sh b/TestScripts/setenv-macos.sh
index 03c9ee26..072c956f 100755
--- a/TestScripts/setenv-macos.sh
+++ b/TestScripts/setenv-macos.sh
@@ -12,6 +12,12 @@
 # licensed under the Boost Software License 1.0, while the individual files
 # in the compilation are all public domain.
 #
+# cpp is set to Apple's cpp. Actually, cpp is merely on-path so Apple's cpp
+# is used. But Apple's cpp is sufficiently different from GNU's cpp and causes
+# Autotools a lot of trouble because Autotools tests are predicated on GNU cpp.
+# If your Autotools project results in "configure:6560: error: C preprocessor
+# cpp fails sanity check", then file a bug report with Autotools.
+#
 # See http://www.cryptopp.com/wiki/MacOS_(Command_Line) for more details
 #############################################################################