folly: switch to FetchContent

Author: JakeHillion

.gitmodules

@@ -1,6 +1,3 @@
 [submodule "extern/drgn"]
   path = extern/drgn
   url = https://github.com/ajor/drgn.git
-[submodule "extern/folly"]
-  path = extern/folly
-  url = https://github.com/JakeHillion/folly.git

CMakeLists.txt

@@ -112,6 +112,16 @@ add_custom_target(librocksdb ALL
 )
 include_directories(SYSTEM "${rocksdb_SOURCE_DIR}/include")
 
+### folly
+### use folly as a header only library. some features won't be supported.
+FetchContent_Declare(
+    folly
+    GIT_REPOSITORY https://github.com/JakeHillion/folly.git
+    GIT_TAG        8db54418e3ccdd97619ac8b69bb3702f82bb0f66
+)
+FetchContent_Populate(folly)
+include_directories(SYSTEM ${folly_SOURCE_DIR})
+
 ### bison & flex (for oid_parser)
 find_package(BISON 3.5 REQUIRED)
 find_package(FLEX)
@@ -147,10 +157,6 @@ find_package(msgpack REQUIRED CONFIG)
 get_target_property(MSGPACK_INCLUDE_DIRS msgpackc INTERFACE_INCLUDE_DIRECTORIES)
 include_directories(SYSTEM ${MSGPACK_INCLUDE_DIRS})
 
-### folly
-### use folly as a header only library. some features won't be supported.
-include_directories(SYSTEM ${CMAKE_CURRENT_SOURCE_DIR}/extern/folly)
-
 ### zstd (for rocksdb)
 find_package(zstd REQUIRED)
 

extern/folly

@@ -45,7 +45,7 @@ containers = [
 
 [headers]
 user_paths = [
-    "../extern/folly",
+    "_deps/folly-src", # this path might be flaky, but it's unlikely to change
 ]
 system_paths = [
     "/usr/include/c++/11",