Compile the check for ICONV_CONST as C++

On OpenIndiana (SunOS) the result is different when compiled as C,
and the C++ result is what matters to wxWidgets.

Fixes compilation of src/common/strconv.cpp on OpenIndiana.

configure

@@ -28118,6 +28118,12 @@ ac_compiler_gnu=$ac_cv_c_compiler_gnu
 if test "$TOOLKIT" != "MSW"; then
 
 if test "$wxUSE_LIBICONV" != "no" ; then
+    ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
 
 
 
@@ -28154,7 +28160,7 @@ iconv_t cd = iconv_open("","");
   return 0;
 }
 _ACEOF
-if ac_fn_c_try_link "$LINENO"; then :
+if ac_fn_cxx_try_link "$LINENO"; then :
   am_cv_func_iconv=yes
 fi
 rm -f core conftest.err conftest.$ac_objext \
@@ -28176,7 +28182,7 @@ iconv_t cd = iconv_open("","");
   return 0;
 }
 _ACEOF
-if ac_fn_c_try_link "$LINENO"; then :
+if ac_fn_cxx_try_link "$LINENO"; then :
   am_cv_lib_iconv=yes
         am_cv_func_iconv=yes
 fi
@@ -28220,7 +28226,7 @@ main ()
   return 0;
 }
 _ACEOF
-if ac_fn_c_try_compile "$LINENO"; then :
+if ac_fn_cxx_try_compile "$LINENO"; then :
   wx_cv_func_iconv_const="no"
 else
   wx_cv_func_iconv_const="yes"
@@ -28249,6 +28255,12 @@ _ACEOF
   fi
 
 
+    ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
     LIBS="$LIBICONV $LIBS"
 fi
 

configure.in

@@ -4117,7 +4117,9 @@ if test "$TOOLKIT" != "MSW"; then
 
 dnl check for available version of iconv()
 if test "$wxUSE_LIBICONV" != "no" ; then
+    AC_LANG_PUSH(C++)
     AM_ICONV
+    AC_LANG_POP()
     LIBS="$LIBICONV $LIBS"
 fi