"Namespace" CMake target names to be able to build Umpire via CMake's FetchContent

[evaleev]

When trying to build Umpire using CMake's FetchContent module from within a large project we run into many target name clashes, e.g.

CMake Error at cmake-build-debug/_deps/umpire-src/blt/cmake/SetupDocs.cmake:9 (add_custom_target):
  add_custom_target cannot create target "docs" because another target with
  the same name already exists.  The existing target is a custom target
  created in source directory
  "/Users/efv/code/tiledarray/cmake-build-debug/_deps/parsec-src/docs".  See
  documentation for policy CMP0002 for more details.
Call Stack (most recent call first):
  cmake-build-debug/_deps/umpire-src/blt/SetupBLT.cmake:141 (include)
  cmake-build-debug/_deps/umpire-src/CMakeLists.txt:77 (include)


CMake Error at cmake-build-debug/_deps/umpire-src/blt/cmake/SetupCodeChecks.cmake:9 (add_custom_target):
  add_custom_target cannot create target "check" because another target with
  the same name already exists.  The existing target is a custom target
  created in source directory "/Users/efv/code/madness".  See documentation
  for policy CMP0002 for more details.
Call Stack (most recent call first):
  cmake-build-debug/_deps/umpire-src/blt/SetupBLT.cmake:146 (include)
  cmake-build-debug/_deps/umpire-src/CMakeLists.txt:77 (include)

To solve this need to "namespace" all CMake targets, e.g. docs should become docs-blt, etc.