Don't hardcode include paths in makefile.am, use $(srcdir) instead (issue #612)

platform/unix/gen-makefile

@@ -1,7 +1,7 @@
 #!/bin/bash
 echo Generating src/Makefile.am ...
 cd src
-inc_current=.
+inc_current='$(srcdir)'
 inc_modules="$inc_current/modules"
 inc_libraries="$inc_current/libraries"
 echo "AM_CPPFLAGS = -I$inc_current -I$inc_modules -I$inc_libraries -I/usr/include/AL -I/usr/include/freetype2  \$(INCLUDE_LUA) -I/usr/include/SDL \$(FILE_OFFSET) -I/usr/include/gme