build: use C++ compiler for linking

[gjasny]

When building with Clang and enabled Sanitizer for Undefined Behavior it is important to use the C++ (and not the C) compiler to drive linking.

Otherwise linking fails with errors like:

/usr/bin/ld: output/pjsua2-test-x86_64-unknown-linux-gnu/main.o: in function `main':
main.cpp:(.text+0xaa): undefined reference to `__ubsan_vptr_type_cache'

To reproduce use something like that:

export CC="/usr/lib/llvm-14/bin/clang"
export CXX="/usr/lib/llvm-14/bin/clang++"
export CPPFLAGS="$CPPFLAGS -DNDEBUG"
export CXXFLAGS="$CXXFLAGS -stdlib=libstdc++ -std=gnu++17 -m64 -fPIC -O3 -fsanitize=undefined"
export CFLAGS="$CFLAGS -m64 -fPIC -O3 -fsanitize=undefined"
export LDFLAGS="$LDFLAGS -m64 -fsanitize=undefined"

./configure --disable-shared --enable-static --enable-epoll --disable-sound --disable-video --disable-speex-aec --disable-l16-codec --disable-gsm-codec --disable-g7221-codec --disable-speex-codec --disable-ilbc-codec --disable-sdl --disable-ffmpeg --disable-v4l2 --disable-openh264 --disable-opencore-amr --disable-silk --disable-opus --disable-bcg729 --disable-libyuv --disable-libwebrtc
make -C pjsip/build pjsua2-test -j24

[sauwming]

Since the issue only happened when someone is using sanitizer, and the sanitiser setup requires a lot of configuration anyway, why is it not the user's responsibility to ensure that it works?

i.e. why not the user just add one more:
export LD=$CXX

[gjasny]

Because the error is within the pjproject realm. You must not use a C compiler to link C++ code.