cmake: Only provide USE_NSEC if struct stat members are avilable.

This allows us to remove OS checks from source code, instead relying
on CMake to detect whether or not `struct stat` has the nanoseconds
members we rely on.

CMakeLists.txt

@@ -19,9 +19,13 @@ CMAKE_POLICY(SET CMP0015 NEW)
 SET(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules/")
 
 INCLUDE(CheckLibraryExists)
+INCLUDE(CheckStructHasMember)
 INCLUDE(AddCFlagIfSupported)
 INCLUDE(FindPkgConfig)
 
+CHECK_STRUCT_HAS_MEMBER("struct stat" st_mtim.tv_nsec sys/stat.h
+	HAVE_STRUCT_STAT_NSEC LANGUAGE C)
+
 # Build options
 #
 OPTION( SONAME				"Set the (SO)VERSION of the target"		ON  )
@@ -37,7 +41,9 @@ OPTION( LIBGIT2_FILENAME	"Name of the produced binary"			OFF )
 OPTION( USE_ICONV			"Link with and use iconv library" 		OFF )
 OPTION( USE_SSH				"Link with libssh to enable SSH support" ON )
 OPTION( USE_GSSAPI			"Link with libgssapi for SPNEGO auth"   OFF )
-OPTION( USE_NSEC			"Care about sub-second file mtimes and ctimes"	OFF )
+IF(HAVE_STRUCT_STAT_NSEC)
+	OPTION( USE_NSEC		"Care about sub-second file mtimes and ctimes"	OFF )
+ENDIF()
 OPTION( VALGRIND			"Configure build for valgrind"			OFF )
 OPTION( CURL			"User curl for HTTP if available" ON)
 

src/index.c

@@ -858,8 +858,7 @@ void git_index_entry__init_from_stat(
 {
 	entry->ctime.seconds = (git_time_t)st->st_ctime;
 	entry->mtime.seconds = (git_time_t)st->st_mtime;
-#if !defined(GIT_WIN32) && !defined(__APPLE__)
-	/* Apple and Windows doesn't provide these struct stat fields. */
+#if defined(GIT_USE_NSEC)
 	entry->mtime.nanoseconds = st->st_mtim.tv_nsec;
 	entry->ctime.nanoseconds = st->st_ctim.tv_nsec;
 #endif

tests/merge/workdir/dirty.c

@@ -164,8 +164,7 @@ static void hack_index(char *files[])
 
 		entry->ctime.seconds = (git_time_t)statbuf.st_ctime;
 		entry->mtime.seconds = (git_time_t)statbuf.st_mtime;
-#if !defined(GIT_WIN32) && !defined(__APPLE__)
-		/* Apple and Windows doesn't provide these struct stat fields. */
+#if defined(GIT_USE_NSEC)
 		entry->ctime.nanoseconds = statbuf.st_ctim.tv_nsec;
 		entry->mtime.nanoseconds = statbuf.st_mtim.tv_nsec;
 #else