allow make even without opengl

CMakeLists.txt

@@ -102,6 +102,7 @@ endif(ENABLE_PNG)
 
 include_directories(${EXTERNAL_INCLUDES})
 
+if(OPENGL_FOUND)
   add_library(lib STATIC gl2ps.c gl2ps.h)
   set_target_properties(lib PROPERTIES OUTPUT_NAME gl2ps)
 
@@ -112,13 +113,15 @@ if(MSVC)
     set_target_properties(shared PROPERTIES COMPILE_FLAGS "-DGL2PSDLL -DGL2PSDLL_EXPORTS")
   endif(MSVC)
 
+  install(TARGETS lib shared DESTINATION lib)
+endif(OPENGL_FOUND)
+
 if(WIN32)
   set(GL2PS_DOC .)
 else(WIN32)
   set(GL2PS_DOC share/doc/gl2ps)
 endif(WIN32)
 
-install(TARGETS lib shared DESTINATION lib)
 install(FILES gl2ps.h DESTINATION include)
 install(FILES ${CMAKE_SOURCE_DIR}/README.txt DESTINATION ${GL2PS_DOC})
 install(FILES ${CMAKE_SOURCE_DIR}/COPYING.LGPL DESTINATION ${GL2PS_DOC})