diff --git a/autoconf/configure.ac b/autoconf/configure.ac
index f9c365cb1c3..45f2fe485ec 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -61,8 +61,8 @@ fi
 
 dnl Default to empty (i.e. assigning the null string to) CFLAGS and CXXFLAGS,
 dnl instead of the autoconf default (for example, '-g -O2' for CC=gcc).
-${CFLAGS=}
-${CXXFLAGS=}
+: ${CFLAGS=}
+: ${CXXFLAGS=}
 
 dnl We need to check for the compiler up here to avoid anything else
 dnl starting with a different one.
diff --git a/configure b/configure
index f3a6594963b..9090cdab5a5 100755
--- a/configure
+++ b/configure
@@ -1992,8 +1992,8 @@ echo "$as_me: error: Already configured in ${srcdir}" >&2;}
   fi
 fi
 
-${CFLAGS=}
-${CXXFLAGS=}
+: ${CFLAGS=}
+: ${CXXFLAGS=}
 
 ac_ext=c
 ac_cpp='$CPP $CPPFLAGS'