build system: check for and turn off some new warnings from clang++ 3.6

These warnings include -Winconsistent-missing-override and
-Wpotentially-evaluated-expression which are both triggered by code out
of my control (Qt and wxWidgets).

Rakefile

@@ -94,6 +94,7 @@ def setup_globals
   $po_multiple_sources     = %{sv}
 
   cflags_common            = "-Wall -Wno-comment -Wfatal-errors #{c(:WLOGICAL_OP)} #{c(:WNO_MISMATCHED_TAGS)} #{c(:WNO_SELF_ASSIGN)} #{c(:QUNUSED_ARGUMENTS)}"
+  cflags_common           += " #{c(:WNO_INCONSISTENT_MISSING_OVERRIDE)} #{c(:WNO_POTENTIALLY_EVALUATED_EXPRESSION)}"
   cflags_common           += " #{c(:OPTIMIZATION_CFLAGS)} -D_FILE_OFFSET_BITS=64 #{c(:MATROSKA_CFLAGS)} #{c(:EBML_CFLAGS)} #{c(:EXTRA_CFLAGS)} #{c(:DEBUG_CFLAGS)} #{c(:PROFILING_CFLAGS)} #{c(:USER_CPPFLAGS)}"
   cflags_common           += " -DMTX_LOCALE_DIR=\\\"#{c(:localedir)}\\\" -DMTX_PKG_DATA_DIR=\\\"#{c(:pkgdatadir)}\\\" -DMTX_DOC_DIR=\\\"#{c(:docdir)}\\\""
   cflags_common           += " -mno-ms-bitfields -DWINVER=0x0500 -D_WIN32_WINNT=0x0500 " if c?(:MINGW)

ac/compiler_flags.m4

@@ -8,6 +8,18 @@ if test x"$ac_cv_compiler_is_clang" = xyes; then
   WNO_SELF_ASSIGN="-Wno-self-assign"
   WNO_MISMATCHED_TAGS="-Wno-mismatched-tags"
 
+  ac_save_CXXFLAGS="$CXXFLAGS"
+  AC_LANG_PUSH(C++)
+
+  CXXFLAGS="$ac_save_CXXFLAGS -Werror -Wno-inconsistent-missing-override"
+  AC_TRY_COMPILE([],[],[ WNO_INCONSISTENT_MISSING_OVERRIDE="-Wno-inconsistent-missing-override" ],[])
+
+  CXXFLAGS="$ac_save_CXXFLAGS -Werror -Wno-potentially-evaluated-expression"
+  AC_TRY_COMPILE([],[],[ WNO_POTENTIALLY_EVALUATED_EXPRESSION="-Wno-potentially-evaluated-expression" ],[])
+
+  AC_LANG_POP()
+  CXXFLAGS="$ac_save_CXXFLAGS"
+
 elif check_version 4.8.0 $ac_cv_gcc_version ; then
   WLOGICAL_OP="-Wlogical-op"
 fi
@@ -16,3 +28,5 @@ AC_SUBST(QUNUSED_ARGUMENTS)
 AC_SUBST(WNO_SELF_ASSIGN)
 AC_SUBST(WNO_MISMATCHED_TAGS)
 AC_SUBST(WLOGICAL_OP)
+AC_SUBST(WNO_INCONSISTENT_MISSING_OVERRIDE)
+AC_SUBST(WNO_POTENTIALLY_EVALUATED_EXPRESSION)

build-config.in

@@ -120,7 +120,9 @@ USER_CFLAGS = @USER_CFLAGS@
 USER_LDFLAGS = @USER_LDFLAGS@
 VORBIS_LIBS = @VORBIS_LIBS@
 WLOGICAL_OP = @WLOGICAL_OP@
+WNO_INCONSISTENT_MISSING_OVERRIDE = @WNO_INCONSISTENT_MISSING_OVERRIDE@
 WNO_MISMATCHED_TAGS = @WNO_MISMATCHED_TAGS@
+WNO_POTENTIALLY_EVALUATED_EXPRESSION = @WNO_POTENTIALLY_EVALUATED_EXPRESSION@
 WNO_SELF_ASSIGN = @WNO_SELF_ASSIGN@
 WXWIDGETS_CFLAGS = @WXWIDGETS_CFLAGS@
 WXWIDGETS_INCLUDES = @WXWIDGETS_INCLUDES@