configure.ac: Fix compilation of Haiku's C++ convenience library

configure.ac: Fix compilation of Haiku's C++ convenience library

Commit 9a1bc8cafb ("build: Require C11 to build and clean up
autoconfig/automake files") added the language standard compiler option
to the AM_CFLAGS and AM_CXXFLAGS. Placing it in the latter is incorrect
as compiling C++ source with the C11 language standard does not make
sense.

Fix this by determining which C11 dialect (GNU or C) the compiler
supports and then constructing the compiler option from that.

Also restrict LT_LDFLAGS to the final libusb library (as was done
previously) since libtool complains about versioning options for
convenience libraries.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>