pulseaudio: fix build error on toolchains without C++ support

pulseaudio: fix build error on toolchains without C++ support

The optional webrtc echo canceller is written in C++, causing auto* to want
to link module-echo-cancel.so with CXX even if webrtc ISN'T used.
If we don't have C++ support enabled in BR, CXX will point to /bin/false,
which makes configure think we aren't able to create C++ .so files
(arguable true), breaking the build when it tries to install the .so
workaround it by patching up the libtool invocations to use C mode instead.

Fixes http://autobuild.buildroot.net/results/114e5bf0086f2394b36f17a70cb54b357a72413c

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>