Use -Wold-style-cast for C++ if supported

Also separate C and C++ warning flags.

autoconf.mk.in

@@ -13,12 +13,13 @@ htmldir=@htmldir@
 pdfdir=@pdfdir
 CC=@CC@
 WFLAGS=@WFLAGS@
+CXXWFLAGS=@CXXWFLAGS@
 CFLAGS=@CFLAGS@ $(WFLAGS)
 LDFLAGS=@LDFLAGS@
 LIBS=@LIBS@
 CPPFLAGS=@CPPFLAGS@
 CXX=@CXX@
-CXXFLAGS=@CXXFLAGS@ $(WFLAGS)
+CXXFLAGS=@CXXFLAGS@ $(CXXWFLAGS) $(WFLAGS)
 AR=@AR@
 RANLIB=@RANLIB@
 DLLTOOL=@DLLTOOL@

configure.ac

@@ -176,6 +176,7 @@ AC_ARG_WITH(buildrules,
 AC_MSG_RESULT($BUILDRULES)
 
 AC_SUBST(WFLAGS)
+AC_SUBST(CXXWFLAGS)
 qpdf_USE_EXTRA_WARNINGS=0
 if test "$BUILDRULES" = "msvc"; then
    dnl /w14267 makes warning 4267 a level 1 warning.  This warning reports
@@ -197,6 +198,22 @@ if test "$qpdf_USE_EXTRA_WARNINGS" = "1"; then
 else
    AC_MSG_RESULT(no)
 fi
+if test "$BUILDRULES" != "msvc"; then
+   qpdf_USE_EXTRA_WARNINGS=0
+   try_flags="-Wold-style-cast"
+   AC_MSG_CHECKING(for whether $CXX supports $try_flags)
+   oCXXFLAGS=$CXXFLAGS
+   CXXFLAGS="$CXXFLAGS $try_flags"
+   AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[int a = 1; int b = a; a = b;]])],
+                     [qpdf_USE_EXTRA_WARNINGS=1],[qpdf_USE_EXTRA_WARNINGS=0])
+   CXXFLAGS=$oCXXFLAGS
+   if test "$qpdf_USE_EXTRA_WARNINGS" = "1"; then
+      AC_MSG_RESULT(yes)
+      CXXWFLAGS="$try_flags"
+   else
+      AC_MSG_RESULT(no)
+   fi
+fi
 
 if test "$BUILDRULES" = "msvc"; then
    try_flags="/WX"

make/libtool.mk

@@ -73,7 +73,7 @@ endef
 # Usage: $(call libcompile,src,includes)
 define c_libcompile
 	$(LIBTOOL) --quiet --mode=compile \
-		$(CC) $(CXXFLAGS) \
+		$(CC) $(CFLAGS) \
 		$(call libdepflags,$(basename $(call c_src_to_obj,$(1)))) \
 		$(foreach I,$(2),-I$(I)) \
 		$(CPPFLAGS) \