COMP: do not export package on build tree

export(PACKAGE VegaFEM) was resulting in Vega being registered
in a CMake Package Registry :
https://cmake.org/Wiki/CMake/Tutorials/Package_Registry

Because of this, an incorrect version of VEGA could be directly
linked against when doing find_package(VegaFEM).

This should only be done when creating a package for installation:
https://cmake.org/cmake/help/git-master/manual/cmake-packages.7.html#creating-packages

CMakeLists.txt

@@ -195,7 +195,7 @@ if (VegaFEM_ENABLE_OpenGL_SUPPORT)
 endif()
 
 add_subdirectory(src)
-export(PACKAGE VegaFEM)
+
 export(TARGETS ${VegaFEM_Modules}
   FILE "${CMAKE_CURRENT_BINARY_DIR}/VegaFEM/VegaFEMTargets.cmake"
   NAMESPACE VegaFEM::