Check existence of mysql_version.h.

On RHEL6.4 or similars mysql_config is included in mysql package,
not in mysql-devel. So it's not enough only checking mysql_config,
we should also check header files.

Although it will be nice to replace checking mysql with AX_LIB_MYSQL
or something, I don't do it in this commit because it's out of scope
of this issue.
Use AC_TRY_COMPILE to detect mysql_version.h

Sorry, using "mysql_config --variable=pkgincludedir" to detect
mysql_version.h isn't suitable because "--variable" option doesn't
exist in default package of CentOS6/ScientificLinux6.
I used MySQL-5.5 to test previous commit.
Merge pull request #12 from ashie/check-mysql-version-h

Check existence of mysql_version.h.