cmake: report whether we are using sub-second stat information

Depending on the platform and on build options, we may or may not
build libgit2 with support for nanoseconds when using `stat`
calls. It's currently unclear though whether sub-second stat
information is used at all.

Add feature info for this to tell at configure time whether it's
being used or not.

cmake/Modules/FindStatNsec.cmake

+INCLUDE(FeatureSummary)
+
 CHECK_STRUCT_HAS_MEMBER ("struct stat" st_mtim "sys/types.h;sys/stat.h"
 	HAVE_STRUCT_STAT_ST_MTIM LANGUAGE C)
 CHECK_STRUCT_HAS_MEMBER ("struct stat" st_mtimespec "sys/types.h;sys/stat.h"
@@ -17,4 +19,8 @@ ENDIF()
 
 IF (HAVE_STRUCT_STAT_NSEC OR WIN32)
 	OPTION( USE_NSEC		"Care about sub-second file mtimes and ctimes"	ON  )
+ELSE()
+	SET(USE_NSEC OFF)
 ENDIF()
+
+ADD_FEATURE_INFO(nanoseconds USE_NSEC "whether to use sub-second file mtimes and ctimes")