chore: More reliable definition of C standard in CMakeLists (#605)

Some local update on my local machine resulted in nanoarrow not
compiling because clang gave a warning for the C11 extensions in the
flatcc generated code. I believe this was because the C standard had
already been set to C99 at the beginning of the file, whereas the check
to update it to C11 only happened after this had already been set.

CMakeLists.txt

@@ -18,11 +18,6 @@
 message(STATUS "Building using CMake version: ${CMAKE_VERSION}")
 cmake_minimum_required(VERSION 3.14)
 
-if(NOT DEFINED CMAKE_C_STANDARD)
- set(CMAKE_C_STANDARD 99)
- set(CMAKE_C_STANDARD_REQUIRED ON)
-endif()
-
 set(NANOARROW_VERSION "0.6.0-SNAPSHOT")
 string(REGEX MATCH "^[0-9]+\\.[0-9]+\\.[0-9]+" NANOARROW_BASE_VERSION
  "${NANOARROW_VERSION}")
@@ -59,6 +54,18 @@ option(NANOARROW_CODE_COVERAGE "Enable coverage reporting" OFF)
 option(NANOARROW_ARROW_STATIC
  "Use a statically-linked Arrow C++ build when linking tests" OFF)
 
+if(NOT DEFINED CMAKE_C_STANDARD)
+ if(NANOARROW_IPC)
+ # flatcc requires C11 for alignas() and static_assert() in flatcc_generated.h
+ # It may be possible to use C99 mode to build the runtime and/or generated header
+ # should this cause problems for users.
+ set(CMAKE_C_STANDARD 11)
+ else()
+ set(CMAKE_C_STANDARD 99)
+ endif()
+ set(CMAKE_C_STANDARD_REQUIRED ON)
+endif()
+
 if(NANOARROW_NAMESPACE)
  set(NANOARROW_NAMESPACE_DEFINE "#define NANOARROW_NAMESPACE ${NANOARROW_NAMESPACE}")
 else()
@@ -150,14 +157,6 @@ target_include_directories(nanoarrow
 install(FILES ${NANOARROW_INSTALL_HEADERS} DESTINATION include/nanoarrow)
 
 if(NANOARROW_IPC)
- # flatcc requires C11 for alignas() and static_assert() in flatcc_generated.h
- # It may be possible to use C99 mode to build the runtime and/or generated header
- # should this cause problems for users.
- if(NOT DEFINED CMAKE_C_STANDARD)
- set(CMAKE_C_STANDARD 11)
- set(CMAKE_C_STANDARD_REQUIRED ON)
- endif()
-
  # Add the flatcc (runtime) dependency
  set(FLATCC_RTONLY
  ON