fix EXTERNAL_LIBS in CMakeLists.txt

[yu239-zz]

The variable 'EXTERNAL_LIB' in the original CMakeLists.txt seems not be used by any other cmake file. I believe that it should be 'EXTERNAL_LIBS'. I did have a CUDA computation issue with the current version of CMakeLists.txt. After changing it to 'EXTERNAL_LIBS', the issue seems to be fixed.

[skylian]

Let me elaborate the "CUDA computation issue". If we don't change EXTERNAL_LIB to EXTERNAL_LIBS, we encountered two problems when running multiple threads that share the same CUDA stream:

1. the numerical results are not correct
2. With higher concurrency, after some time, the threads will hang at cudaStreamSynchronize's call clock_gettime(), and GPU utility will be 100% all the time.

One question is, if we don't correctly provide the CUDA runtime libraries to Paddle in cmake configuration, why can we still successfully build and run Paddle?

[skylian]

I'm sorry. Please ignore my comment above. We cannot replicate the problem by changing back to EXTERNAL_LIB.

[gangliao]

LGTM