[CMake] ExternalProject and install dir

[David Cole]

If x is a CMake-driven project, you'll also need to explicitly set 
CMAKE_INSTALL_PREFIX when configuring. If not, there's likely a 
"--prefix" arg for configuring... One of those also has to be set to 
install to a non-default location.

The PREFIX arg for ExternalProject is only used to organize the 
source/build/install directory values for the custom target. It's still 
up to you to configure and build it properly for a non-default install 
location.


HTH,
David C.