Compile OMPlot with C++11

C++11 is required in Qt 5.7

OMPlot/OMPlotGUI/OMPlotGUI.config.in

@@ -6,5 +6,5 @@ LIBS += -L @OMBUILDDIR@/lib/@host_short@/omc -lomqwt @RPATH_QMAKE@
 INCLUDEPATH += @OPENMODELICAHOME@/include/omplot/qwt @OPENMODELICAHOME@/include/omc/c
 
 QMAKE_CFLAGS = @CFLAGS@ @CPPFLAGS@
-QMAKE_CXXFLAGS = @CFLAGS@ @CPPFLAGS@
+QMAKE_CXXFLAGS = @CXXFLAGS@ @CPPFLAGS@
 QMAKE_LFLAGS += @LDFLAGS@

configure.ac

@@ -12,6 +12,7 @@ AC_SUBST(host_short)
 
 m4_include([common/m4/ombuilddir.m4])
 m4_include([common/m4/pre-commit.m4])
+m4_include([common/m4/ax_cxx_compile_stdcxx_11.m4])
 cp common/install-sh common/config.guess common/config.sub ./
 
 AC_CANONICAL_BUILD
@@ -21,9 +22,11 @@ AC_CANONICAL_TARGET
 rm install-sh
 
 dnl Checks for programs.
+test -z "$CXXFLAGS" && CXXFLAGS="$CFLAGS"
 AC_LANG([C++])
 AC_PROG_CC
 AC_PROG_CXX
+AX_CXX_COMPILE_STDCXX_11([noext],[mandatory])
 
 host_short=$host_cpu-$host_os
 

qwt/Makefile.unix.in

@@ -14,7 +14,7 @@ all: build
 
 Makefile: qwt.pro
 	@rm -f $@
-	$(QMAKE) QMAKE_CXX=@CXX@ qwt.pro
+	$(QMAKE) QMAKE_CXX=@CXX@ QMAKE_CXXFLAGS="@CXXFLAGS@" qwt.pro
 clean:
 	test ! -f Makefile || $(MAKE) -f Makefile clean
 	rm -rf build lib Makefile