build: Enable compiling with -Werror.

This helps find compile warnings because they simply break
the build.

To use:
./configure --enable-warnings --enable-werror

Signed-off-by: Ben Greear <greearb@candelatech.com>

Makefile.am

@@ -99,6 +99,9 @@ endif
 
 libcares_la_LDFLAGS = $(UNDEF) $(VER)
 
+# Add -Werror if defined
+CFLAGS += @CARES_CFLAG_EXTRAS@
+
 # Makefile.inc provides the CSOURCES and HHEADERS defines
 include Makefile.inc
 

configure.ac

@@ -13,6 +13,7 @@ AM_MAINTAINER_MODE
 CARES_CHECK_OPTION_DEBUG
 CARES_CHECK_OPTION_OPTIMIZE
 CARES_CHECK_OPTION_WARNINGS
+CARES_CHECK_OPTION_WERROR
 CARES_CHECK_OPTION_CURLDEBUG
 CARES_CHECK_OPTION_SYMBOL_HIDING
 
@@ -862,6 +863,12 @@ CARES_CONFIGURE_SYMBOL_HIDING
 CARES_PRIVATE_LIBS="$LIBS"
 AC_SUBST(CARES_PRIVATE_LIBS)
 
+CARES_CFLAG_EXTRAS=""
+if test X"$want_werror" == Xyes; then
+  CARES_CFLAG_EXTRAS="-Werror"
+fi
+AC_SUBST(CARES_CFLAG_EXTRAS)
+
 dnl squeeze whitespace out of some variables
 
 squeeze CFLAGS

m4/cares-confopts.m4

@@ -253,6 +253,37 @@ AC_HELP_STRING([--disable-warnings],[Disable strict compiler warnings]),
   AC_MSG_RESULT([$want_warnings])
 ])
 
+dnl CARES_CHECK_OPTION_WERROR
+dnl -------------------------------------------------
+dnl Verify if configure has been invoked with option
+dnl --enable-werror or --disable-werror, and set
+dnl shell variable want_werror as appropriate.
+
+AC_DEFUN([CARES_CHECK_OPTION_WERROR], [
+  AC_MSG_CHECKING([whether to treat compile warnings as errors])
+  OPT_COMPILER_WERROR="default"
+  AC_ARG_ENABLE(werror,
+AC_HELP_STRING([--enable-werror],[Treat compile warnings as errors])
+AC_HELP_STRING([--disable-warnings],[Don't treat compile warnings as werrors]),
+  OPT_COMPILER_WERROR=$enableval)
+  case "$OPT_COMPILER_WERROR" in
+    no)
+      dnl --disable-werror option used
+      want_werror="no"
+      ;;
+    default)
+      dnl configure option not specified, so
+      dnl use same setting as --enable-debug
+      want_werror="no"
+      ;;
+    *)
+      dnl --enable-werror option used
+      want_werror="yes"
+      ;;
+  esac
+  AC_MSG_RESULT([$want_werror])
+])
+
 
 dnl CARES_CHECK_NONBLOCKING_SOCKET
 dnl -------------------------------------------------