Static libraries have to be built as position-independent, otherwise
they cannot be linked into a shared library which is built
position-independent (-fPIC).

Without this, linking objects into libcurl.so fails with:

    /usr/bin/ld: libngtcp2.a(ngtcp2_conn.c.o): relocation R_X86_64_PC32 against symbol `__asan_option_detect_stack_use_after_return' can not be used when making a shared object; recompile with -fPIC

Fix detection of proper OpenSSL and GnuTLS libraries

Until mainline support of QUIC are present in OpenSSL and GnuTLS, be
sure to test whether the found library versions are the patched ones
since the version number check is insufficient. This fixes the default
build on a system where a new GnuTLS is present without the patches from
the GnuTLS tmp-quic branch.

Add new ENABLE_OPENSSL and ENABLE_GNUTLS CMake options to allow
explicitly disabling these libraries to mirror the --without-openssl and
the --without-gnutls functionality of autotools.

Partially addresses https://github.com/ngtcp2/ngtcp2/issues/238

Fix integer wraparound on 32-bit machines

Remove unused errno.h include from ngtcp2_log

This causes conn_get_earliest_pktns() to falsely report a nonzero loss time, resulting in spurious retransmissions.

Commit 0999d327 ("Pass user-defined
printf function rather than passing file descriptor") removed the usage
of errno so it does not need to be included.

Remove unused string.h include from examplestest

Remove ngtcp2_conn_get_bytes_in_flight.  Use ngtcp2_conn_get_conn_stat
instead.

Fix assertion failure which occurs when CWND is filled up with
Handshake probe packet and Short packet in the first call of
ngtcp2_conn_writev_stream with NGTCP2_WRITE_STREAM_FLAG_MORE flag set.
The next ngtcp2_conn_writev_stream gets assertion failure.

Fix format specifiers and missing argument passed to log_printf()

Fix format specifier in log_fr_connection_close()

ngtcp2_connection_close.reason_len is size_t, which is not necessarily 64-bit.