compile embedded with PIC to allow linking with shared libraries

Following autotools tradition, add preprocessor definitions  specified in environment variable CPPFLAGS

Fix failing sys_vars.plugin_dir_basic - treat both '/' and '\' as path separators in get_relative_path() on Windows

using  cmake option INSTALL_LAYOUT=STANDALONE would produce the layout as in 
tar.gz or zip packages.
INSTALL_LAYOUT=UNIX will produce unixish install layout  (with mysqld being in sbin subdirectory , libs in lib/mysql etc). This layout is used for RPM packages.

Subtle differences in both packages unfortunately lead to the need to recompile MySQL to use with other package type - as otherwise for example default plugins or data directories would be wrong set.

There are numerous other variables that  allow fine-tuning packaging layout. (INSTALL_BINDIR, INSTALL_LIBDIR , INSTALL_PLUGINDIR etc).

This options are different from autotools as they do not expect full paths to directories, but only subdirectory of CMAKE_INSTALL_PREFIX.

There are 2 special options that expect full directory paths
- MYSQL_DATADIR that defines default MYSQL data directory (autotools equivalent
is --localstatedir)
- SYSCONFDIR  can be added to search my.cnf search path (autotools equivalent is --sysconfdir)

 WORDS_BIGENDIAN was wrongly defined to 1

and later

in errmsg.txt. Kept the one in that matched the location
in errmsg-utf8.txt, ie after ER_UNKNOWN_LOCALE.

boolean variables to ensure option is in cache. Also use documented CMAKE_USE_MAKE_RULES_OVERRIDE feature instead of INCLUDE.

- remove superfluous tests in innobase's CMakeLists.txt

Fix AIX error during compile in my_handler (pointer to data is used, while pointer to function is expected). 
This also has generated warnings on all other platforms

make -DBUILD_CONFIG=mysql_release work

They are probably sometimes used by the bteam

just in case old linux is used, add _GNU_SOURCE preprocessor constant to be able to see pthread_rwlock_t

-lm is sometimes not linked, which causes errors about undefined rint()
- FreeBSD embedded library does not link with the flags returned by mysql_config
 (added -lcrypt to LIBS, whereby causing overlinking in case of normal non-embedded client)

for regular PB builds.

Init MY_PTHREAD_MUTEX_FAST prior to first usage to avoid crash on
FreeBSD