Allow TCPDF to be removed

We currently require TCPDF to be present, but in most cases this is
feature which users do not need and thus can reduce size of installed
phpMyAdmin.

Also this is helpful for Linux distributions as they can provide soft
dependency on tcpdf package.

Signed-off-by: Michal Čihař <michal@cihar.com>
Do not offer PDF export if TCPDF is not present

Signed-off-by: Michal Čihař <michal@cihar.com>
Merge pull request #1092 from nijel/optional-tcpdf

Allow TCPDF to be removed
Fix merge conflict

Signed-off-by: Marc Delisle <marc@infomarc.info>
Merge commit 'dcb18ef'
Merge commit '268ebba'