Use system dynolog headers and system fmt.

Point the ipcfabric include paths at dev-libs/dynolog instead of the
vendored third_party/dynolog_headers, and take fmt from dev-libs/libfmt
instead of the third_party/fmt submodule. The link to fmt::fmt-header-only
stays: libkineto uses fmt as a header-only library, so it neither records
a NEEDED entry for libfmt nor leaves fmt symbols unresolved. ::gentoo's
p20260805 patch also dropped that link, which left libkineto.so with
undefined fmt::v12 symbols that only resolved when the consumer happened
to link libfmt itself.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -238,23 +238,10 @@
 # dynolog's ipcfabric is header-only and vendored under third_party/dynolog_headers
 # (pinned in version.txt), so only its include paths are needed below -- there
 # is no library to build or link.
-set(DYNOLOG_INCLUDE_DIR "${LIBKINETO_THIRDPARTY_DIR}/dynolog_headers/")
-set(IPCFABRIC_INCLUDE_DIR "${DYNOLOG_INCLUDE_DIR}/dynolog/src/ipcfabric/")
+set(DYNOLOG_INCLUDE_DIR "@GENTOO_PORTAGE_EPREFIX@/usr/include/dynolog/")
+set(IPCFABRIC_INCLUDE_DIR "@GENTOO_PORTAGE_EPREFIX@/usr/include/dynolog/src/ipcfabric/")
 
-if(NOT TARGET fmt::fmt-header-only)
-  if(NOT FMT_SOURCE_DIR)
-    set(FMT_SOURCE_DIR "${LIBKINETO_THIRDPARTY_DIR}/fmt"
-      CACHE STRING "fmt source directory from submodules")
-  endif()
-
-  # Build FMT.
-  # FMT and some other libraries use BUILD_SHARED_LIBS to control
-  # the library type.
-  # Save and restore the value after configuring FMT
-  set(FMT_INSTALL OFF)
-  add_subdirectory("${FMT_SOURCE_DIR}" "${LIBKINETO_BINARY_DIR}/fmt")
-  message(STATUS "Kineto: FMT_SOURCE_DIR = ${FMT_SOURCE_DIR}")
-endif()
+find_package(fmt REQUIRED CONFIG)
 
 message(STATUS " DYNOLOG_INCLUDE_DIR = ${DYNOLOG_INCLUDE_DIR}")
 message(STATUS " IPCFABRIC_INCLUDE_DIR = ${IPCFABRIC_INCLUDE_DIR}")