Several places in the code we do compile-time checks of
MYSQL_VERSION_ID without including mysql_version.h where
MYSQL_VERSION_ID is defined.

This patch makes such checks less brittle by:
- Removing them where possible
- Explicitly including mysql_version.h otherwise

The patch also fixes a similar problem where some
files related to MyISAM full text search use the
HA_KEYTYPE_FLOAT symbol without including my_base.h
where it is defined.

These problems were identified by studying the output
from the -Wundef Clang compiler warning option.