Revert "cmake: use writev(3p) wrapper as needed"

This reverts commit 89152af; let's
not use writev() for now.

Author: gitster

contrib/buildsystems/CMakeLists.txt

@@ -376,7 +376,7 @@ endif()
 #function checks
 set(function_checks
 	strcasestr memmem strlcpy strtoimax strtoumax strtoull
-	setenv mkdtemp poll pread memmem writev)
+	setenv mkdtemp poll pread memmem)
 
 #unsetenv,hstrerror are incompatible with windows build
 if(NOT WIN32)
@@ -421,10 +421,6 @@ if(NOT HAVE_MEMMEM)
 	list(APPEND compat_SOURCES compat/memmem.c)
 endif()
 
-if(NOT HAVE_WRITEV)
-	list(APPEND compat_SOURCES compat/writev.c)
-endif()
-
 if(NOT WIN32)
 	if(NOT HAVE_UNSETENV)
 		list(APPEND compat_SOURCES compat/unsetenv.c)