fix install target for non-windows

CMakeLists.txt

@@ -1286,8 +1286,10 @@ if(ENABLE_BUILD_LIB)
   install(TARGETS lib DESTINATION ${GMSH_LIB} OPTIONAL)
 endif(ENABLE_BUILD_LIB)
 if(ENABLE_BUILD_SHARED OR ENABLE_BUILD_DYNAMIC)
-  install(TARGETS shared RUNTIME DESTINATION ${GMSH_LIB} OPTIONAL)
   if(WIN32 AND NOT MSVC OR CYGWIN)
+    # install only the runtime (DLL); the .dll.a generated by gcc is not usable
+    # by MSVC
+    install(TARGETS shared RUNTIME DESTINATION ${GMSH_LIB} OPTIONAL)
     # install .def file
     install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${LIBGMSH_DEF} 
             DESTINATION ${GMSH_LIB})
@@ -1299,6 +1301,8 @@ if(ENABLE_BUILD_SHARED OR ENABLE_BUILD_DYNAMIC)
       message(STATUS "Will install extra DLLs for Windows")
       install(FILES ${GCC_DLL} ${GFORTRAN_DLL} ${STDC_DLL} DESTINATION ${GMSH_LIB})
     endif(GCC_DLL AND GFORTRAN_DLL AND STDC_DLL)
+  else(WIN32 AND NOT MSVC OR CYGWIN)
+    install(TARGETS shared DESTINATION ${GMSH_LIB} OPTIONAL)
   endif(WIN32 AND NOT MSVC OR CYGWIN)
 endif(ENABLE_BUILD_SHARED OR ENABLE_BUILD_DYNAMIC)