add OBJECT property in generic cmake

[gangliao]

cc_library|nv_library(<target_name>
[STATIC SHARED OBJECT]
  SRCS <file>...
  OBJS <objs>...
  DEPS <libs>...)

cc_library and nv_library can generate *.o, *.a, or *.so
if the corresponding keyword OBJECT, STATIC or SHARED is specified.

cc_binary|nv_binary(<target_name>
  SRCS <file>...
  OBJS <objs>...
  DEPS <libs>...)

cc_binary and nv_binary can build source code and link the dependent
libraries to generate a binary.

cc_test|nv_test(<target_name>
  SRCS <file>...
  OBJS <objs>...
  DEPS <libs>...)

cc_test and nv_test can build test code, link gtest and other dependents
libraries to generate the test suite.

For example, in one folder, it contains
  ddim{.h, .cc, _test.cc, _test.cu}
  place{.h, cc, _test.cc}

We can add build script as follows: 

cc_library(place OBJECT
   SRCS place.cc)

place.cc -> place.o
cc_library's OBJECT OPTION will generate place.o.

cc_test(place_test
   SRCS place_test.cc
   OBJS place
   DEPS glog gflags)

place_test.cc, place.o, glog, gflags -> place_test
cc_test will combine place_test.cc, place.o with libglog.a
and libgflags.a to generate place_test.

cc_library(ddim OBJECT
   SRCS ddim.cc)

ddim.cc -> ddim.o
cc_library's OBJECT OPTION will generate ddim.o.

cc_test(ddim_test
   SRCS ddim_test.cc
   OBJS ddim)

ddim_test.cc, ddim.o -> ddim_test
cc_test will build ddim_test.cc with ddim.o to generate ddim_test.

nv_test(dim_test
   SRCS dim_test.cu
   OBJS ddim)

dim_test.cu, ddim.o -> dim_test
nv_test will build dim_test.cu with ddim.o to generate dim_test.

cc_library(majel
   OBJS place ddim)

place.o, ddim.o -> libmajel.a
cc_library's default OPTION is STATIC. It will archive place.o
and ddim.o to generate libmajel.a.

[gangliao]

generate libmajel.a from place and ddim.

[wangkuiyi]

I would highly recommend not to expose the fact that the building system generates .a and .o to end users. I mean let us don't add the OBJECT parameter.

[Xreki]

Maybe we can decide to generate *.o, *.a according to whether there are SRCS. As in your example, there is no SRCS when combining several *.o to a x.a.

cc_library(majel
   OBJS place ddim)