remove unused sql/examples
cmake: prefer INSTALL(PROGRAMS over INSTALL(FILES
cmake cleanup: use MY_CHECK_AND_SET_COMPILER_FLAG.

move MY_CHECK_C_COMPILER_FLAG and MY_CHECK_CXX_COMPILER_FLAG
to a separate file, introduce MY_CHECK_AND_SET_COMPILER_FLAG,
use it where possible.
MDEV-5730 enhance security using special compilation options

-Wl,-z,relro,-z,now
-pie
-fstack-protector --param=ssp-buffer-size=4
-D_FORTIFY_SOURCE=2