Do not reject clang or gcc as unsupported compilers on Solaris.
Note that both compilers are experimental, and cannot currently be used
for production code.

Add misc FLAGS settings for using clang on Solaris.
It uses gcc9 libraries and headers, so we need explicit -Wl,-L... arguments.

Fix a few cmake places where we assume that SunPro is the only compiler
available on Solaris.

Silence some gcc warnings previously only relevant on Linux.

Note that clang is completely unusable at this point,
'static thread_local' variables will cause segfault during atexit processing.
There is a new test executable to prove it.

For non-trivial uses of static thread_local, the compiler will give an error:
storage/temptable/src/allocator.cc:45:32: error:
cannot compile this non-trivial TLS destruction yet
static thread_local End_thread end_thread;

gcc seems to work reasonably well on Intel, except for GIS boost code.
gcc on Sparc works OK for DEBUG builds. Non-DEBUG builds however fail
to bootstrap. The stack is garbled when handle_bootstrap() does:
    if (handler) {
      args->m_bootstrap_error = (*handler)(thd);
when called from Dictionary_impl::init() like this:
  // Creation of Data Dictionary through current server
  if (dd_init == enum_dd_init_type::DD_INITIALIZE)
    result = ::bootstrap::run_bootstrap_thread(
        nullptr, nullptr, &bootstrap::initialize, SYSTEM_THREAD_DD_INITIALIZE);

Tested with clang on Intel:
/bin/clang --version
clang version 6.0.1 (tags/RELEASE_601/final)
uname -a
SunOS xxxxxx 5.11 11.4.15.5.0 i86pc i386 i86pc

Tested with gcc on Intel and Sparc:
/usr/gcc/9/bin/gcc --version
gcc (GCC) 9.2.0
uname -a
SunOS xxxxxx 5.11 11.4.15.5.0 i86pc i386 i86pc
uname -a
SunOS xxxxxx 5.11 11.4.15.5.0 sun4v sparc sun4v

Change-Id: Ia57ef8cd6e448804ece0dbfd66da1695a8e7361f