books/porters-handbook: Add USES=tex macro

In a recent commit I have changed USE_TEX macros to USES=tex in the
ports tree. Till now there were in fact no descriptions of TEX macros at
all. So introduce the new macros here in the porters-handbook.

While I am here remove EOL spaces.

Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D37745