[CAS] LLVM_ENABLE_ONDISK_CAS requires flock on UNIX (#159207)

As a follow up to #159122, after figure out reason why CAS unit tests
are failing on Solaris, update the CMake configuration to build ondisk
CAS implementation. We now check the existance of `flock` before
enabling the configuration.

In the future, we can find ways to support OnDiskCAS on other platforms
that do not have `flock`. This can techinically be done with a POSIX
compilant file lock but that will put a restriction on the usage of CAS.

Author: cachemeifyoucan

llvm/CMakeLists.txt

@@ -879,13 +879,6 @@ option (LLVM_ENABLE_OCAMLDOC "Build OCaml bindings documentation." ON)
 option (LLVM_ENABLE_BINDINGS "Build bindings." ON)
 option (LLVM_ENABLE_TELEMETRY "Enable the telemetry library. If set to OFF, library cannot be enabled after build (eg., at runtime)" ON)
 
-set(LLVM_ENABLE_ONDISK_CAS_default ON)
-if("${CMAKE_SYSTEM_NAME}" MATCHES SunOS)
-  # Build OnDiskCAS by default only on non-Solaris machines.
-  set(LLVM_ENABLE_ONDISK_CAS_default OFF)
-endif()
-option(LLVM_ENABLE_ONDISK_CAS "Build OnDiskCAS." ${LLVM_ENABLE_ONDISK_CAS_default})
-
 set(LLVM_INSTALL_DOXYGEN_HTML_DIR "${CMAKE_INSTALL_DOCDIR}/llvm/doxygen-html"
     CACHE STRING "Doxygen-generated HTML documentation install directory")
 set(LLVM_INSTALL_OCAMLDOC_HTML_DIR "${CMAKE_INSTALL_DOCDIR}/llvm/ocaml-html"

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -1469,3 +1469,11 @@ if(LLVM_ENABLE_LLVM_LIBC)
     message(WARNING "Unable to link against LLVM libc. LLVM will be built without linking against the LLVM libc overlay.")
   endif()
 endif()
+
+check_symbol_exists(flock "sys/file.h" HAVE_FLOCK)
+set(LLVM_ENABLE_ONDISK_CAS_default OFF)
+if(HAVE_FLOCK OR LLVM_ON_WIN32)
+  # LLVM OnDisk CAS currently requires flock on Unix.
+  set(LLVM_ENABLE_ONDISK_CAS_default ON)
+endif()
+option(LLVM_ENABLE_ONDISK_CAS "Build OnDiskCAS." ${LLVM_ENABLE_ONDISK_CAS_default})