Build tests for presence of DR extension .so, not absence of .a

_From [thomasra...@gmail.com](https://code.google.com/u/108998612245273907729/) on March 31, 2013 15:16:09_

[Using r1241 / 4ef259d2 from git://github.com/rnk/drmemory.git]

CMakeLists ensures that dynamorio was built with static extensions, and otherwise calls
message(FATAL_ERROR "${DynamoRIO_DIR} was built with dynamic extensions:
please re-build with static extension libraries")

This is highly misleading. The check triggers on the _presence_ of .so files, not on the _absence_ of .a files like one might think. Therefore it is _not_ enough to rebuild and reinstall with static libraries; one has to nuke the previously installed dynamic libs!

I don't know if the .so need to be absent, but if so, please save the next victim these 10min of digging and fix that message to something more informative. Otherwise, change the test to see if there are any .a to be found. Thanks.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=1179_