configure.ac: fix C++11 support check

[shr-project]

• with -Werror=return-type in CFLAGS this test fails with:

  configure:4290: checking for C++11 support or better configure:4303: x86_64-webos-linux-g++ -m64 -march=core2 -mtune=core2 -msse3 -mfpmath=sse -fstack-protector-strong -O2 -D_FORTIFY_SOURCE=2 -Wformat -Wformat-security -Werror=format-security -Werror=return-type --sysroot=/OE/lge/build/webos/mickledore/BUILD/work/qemux86_64-webos-linux/rdfind/1.5.0-r0/recipe-sysroot -c -O2 -pipe -g -feliminate-unused-debug-types -fmacro-prefix-map=/OE/lge/build/webos/mickledore/BUILD/work/qemux86_64-webos-linux/rdfind/1.5.0-r0/rdfind-1.5.0=/usr/src/debug/rdfind/1.5.0-r0 -fdebug-prefix-map=/OE/lge/build/webos/mickledore/BUILD/work/qemux86_64-webos-linux/rdfind/1.5.0-r0/rdfind-1.5.0=/usr/src/debug/rdfind/1.5.0-r0 -fmacro-prefix-map=/OE/lge/build/webos/mickledore/BUILD/work/qemux86_64-webos-linux/rdfind/1.5.0-r0/build=/usr/src/debug/rdfind/1.5.0-r0 -fdebug-prefix-map=/OE/lge/build/webos/mickledore/BUILD/work/qemux86_64-webos-linux/rdfind/1.5.0-r0/build=/usr/src/debug/rdfind/1.5.0-r0 -fdebug-prefix-map=/OE/lge/build/webos/mickledore/BUILD/work/qemux86_64-webos-linux/rdfind/1.5.0-r0/recipe-sysroot= -fmacro-prefix-map=/OE/lge/build/webos/mickledore/BUILD/work/qemux86_64-webos-linux/rdfind/1.5.0-r0/recipe-sysroot= -fdebug-prefix-map=/OE/lge/build/webos/mickledore/BUILD/work/qemux86_64-webos-linux/rdfind/1.5.0-r0/recipe-sysroot-native= -fvisibility-inlines-hidden conftest.cpp >&5 conftest.cpp: In function 'int f()': conftest.cpp:22:20: error: no return statement in function returning non-void [-Werror=return-type] 22 | int f() { auto a=1;} | ^ cc1plus: some warnings being treated as errors ... configure:4308: error: no c++11 support, please set CXXFLAGS properly

• fix the test to pass

[pauldreik]

thank you, included in main.