Add BUILD_SHARED_LIBS option defaulting to ON (#1702)

This means that downstream libraries that get their `fmt` dependency from RMM will use `-DFMT_SHARED` in the compile command: this matches what the rapids combined devcontainers do, so we get sccache hits.

- Closes #1701

Authors:
  - Lawrence Mitchell (https://github.com/wence-)

Approvers:
  - Kyle Edwards (https://github.com/KyleFromNVIDIA)

URL: #1702

CMakeLists.txt

@@ -41,6 +41,9 @@ rapids_cmake_build_type(Release)
 option(RMM_NVTX "Build RMM with NVTX support" OFF)
 option(BUILD_TESTS "Configure CMake to build tests" ON)
 option(BUILD_BENCHMARKS "Configure CMake to build (google) benchmarks" OFF)
+# This is mostly so that dependent libraries, such as fmt, are configured in shared mode for
+# downstream dependents of RMM that get their common dependencies transitively.
+option(BUILD_SHARED_LIBS "Build RMM shared libraries" ON)
 set(RMM_LOGGING_LEVEL
     "INFO"
     CACHE STRING "Choose the logging level.")