Build system: added test for beamer

git-svn-id: https://www.dynare.org/svn/dynare/trunk@2955 ac1d8469-bf42-47a9-8791-bf33cf982152

Makefile.am

 if HAVE_PDFLATEX
+if HAVE_BEAMER
 pdf-local: preprocessor.pdf
 endif
+endif
 
 SRC = preprocessor.tex expr.png expr-sharing.png matrices.png overview.png