Fix to cmake build for clang

[jczaja]

Failure scenario that is fixed:

1. mkdir mybuild; cd mybuild
2. CXX=/usr/bin/clang++ cmake -DBLAS=open
3. make all; make test
4. make runtest

Root cause of problem:
cmake/Targets.cmake is having a check for BUILD_SHARED_LIBS , but at the moment
of inclusion of mentioned file, such variable does not exist yet.

Solution:
Condition checking for BUILD_SHARED_LIBS was wraped into macro , and invoked
lateron eg. after BUILD_SHARED_LIBS is initialized

[shelhamer]

@Nerei can you check this?

[Nerei]

Hi, @jczaja This doesn't reproduce for me. What is an effect of these repro-steps? Non-registered layers? Shouldn't be. Nonetheless the change looks reasonable. Looks good for me.

[jczaja]

Hi, @Nerei ,

I missed (not intentionally) important detail here. Reproduction is to be achieved only on Linux.

Effect of reproduction is that linking won't be successful. Build won't be successful. This is because if BUILD_SHARED_LIBS is not present at the time of checking then MacOS's Clang specific line will be executed eg.

set(Caffe_LINK -Wl,-force_load caffe)

instead of:

set(Caffe_LINK caffe)

So my Bad that I did not explicitly mentioned that this is Linux specific. It is very likely that it works fine on MacOS.

[Nerei]

Thanks. Anyway good change

[shelhamer]

Thanks for the fix @jczaja and check @Nerei.