diff --git a/source/acinclude.m4 b/source/acinclude.m4
--- a/source/acinclude.m4
+++ b/source/acinclude.m4
@@ -459,30 +459,36 @@ AC_DEFUN([AC_CHECK_STRICT_COMPILE],
       ], [ac_use_strict_options=yes])
     AC_MSG_RESULT($ac_use_strict_options)
 
     if test "$ac_use_strict_options" = yes
     then
         if test "$GCC" = yes
         then
             CFLAGS="$CFLAGS -Wall -pedantic -Wshadow -Wpointer-arith -Wmissing-prototypes -Wwrite-strings"
+
+            # Suppress clang C warnings:
+            CFLAGS="$CFLAGS -Wno-sign-compare -Wno-unused"
         else
             case "${host}" in
             *-*-cygwin)
                 if test "`$CC /help 2>&1 | head -c9`" = "Microsoft"
                 then
                     CFLAGS="$CFLAGS /W4"
                 fi ;;
             *-*-mingw*)
                 CFLAGS="$CFLAGS -W4" ;;
             esac
         fi
         if test "$GXX" = yes
         then
             CXXFLAGS="$CXXFLAGS -W -Wall -pedantic -Wpointer-arith -Wwrite-strings -Wno-long-long"
+
+            # Suppress clang C++ warnings:
+            CXXFLAGS="$CXXFLAGS -Wno-unused -Wno-unused-parameter"
         else
             case "${host}" in
             *-*-cygwin)
                 if test "`$CXX /help 2>&1 | head -c9`" = "Microsoft"
                 then
                     CXXFLAGS="$CXXFLAGS /W4"
                 fi ;;
             *-*-mingw*)
diff --git a/source/configure b/source/configure
--- a/source/configure
+++ b/source/configure
@@ -5227,30 +5227,36 @@ fi
     { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $ac_use_strict_options" >&5
 printf "%s\n" "$ac_use_strict_options" >&6; }
 
     if test "$ac_use_strict_options" = yes
     then
         if test "$GCC" = yes
         then
             CFLAGS="$CFLAGS -Wall -pedantic -Wshadow -Wpointer-arith -Wmissing-prototypes -Wwrite-strings"
+
+            # Suppress clang C warnings:
+            CFLAGS="$CFLAGS -Wno-sign-compare -Wno-unused"
         else
             case "${host}" in
             *-*-cygwin)
                 if test "`$CC /help 2>&1 | head -c9`" = "Microsoft"
                 then
                     CFLAGS="$CFLAGS /W4"
                 fi ;;
             *-*-mingw*)
                 CFLAGS="$CFLAGS -W4" ;;
             esac
         fi
         if test "$GXX" = yes
         then
             CXXFLAGS="$CXXFLAGS -W -Wall -pedantic -Wpointer-arith -Wwrite-strings -Wno-long-long"
+
+            # Suppress clang C++ warnings:
+            CXXFLAGS="$CXXFLAGS -Wno-unused -Wno-unused-parameter"
         else
             case "${host}" in
             *-*-cygwin)
                 if test "`$CXX /help 2>&1 | head -c9`" = "Microsoft"
                 then
                     CXXFLAGS="$CXXFLAGS /W4"
                 fi ;;
             *-*-mingw*)
