build: import fmt using find_package

Since release 1.12 avro uses the fmt library instead of boost::format.
This was introduced in apache@19501fc.

However, we want to avoid pulling in fmt using fetchcontent like the
build currently does and instead we want to use find_package to reuse
the existing fmt dependency in our vtools-based build.

The version is fixed to the current fmt version used by the vtools
build which works with the avro build.

lang/c++/CMakeLists.txt

@@ -82,15 +82,7 @@ endif ()
 find_package (Boost 1.38 REQUIRED
     COMPONENTS filesystem iostreams program_options regex system)
 
-include(FetchContent)
-FetchContent_Declare(
-        fmt
-        GIT_REPOSITORY  https://github.com/fmtlib/fmt.git
-        GIT_TAG         10.2.1
-        GIT_PROGRESS    TRUE
-        USES_TERMINAL_DOWNLOAD TRUE
-)
-FetchContent_MakeAvailable(fmt)
+find_package(fmt 8.1.1 REQUIRED)
 
 find_package(Snappy)
 if (SNAPPY_FOUND)