Add cmake platform checks

- Defined CMake checks for all headers in config.h.in
- Removed headers checks that are not used anymore:
  sgtty.h sys/statfs.h libintl.h poll.h
- Added UNIX check
- Add some fatal checks
  + Check for setenv() and fail if it does not exist
    since our os layer just assumes it does
  + lstat is required by os_unix.c
  + sys/wait.h is required in UNIX
- Removed entries for functions that are not being used: getcwd,
  getrlimit, getwd, nanosleep, sigaltstack, getwd, sigstack, fseeko
- Replaced nearly all defines in config.h.in for functions with
  compile time checks
- Add check for symbol FD_CLOEXEC
- Add check for langinfo CODESET
- HAVE_ICONV_H and HAVE_ICONV hold the expected checks but Neovim uses
  USE_ICONV define to actually decide whether to enable it
- Removed checks that are no longer needed
  + USEMEMMOVE
  + _FILE_OFFSET_BITS
  + HAVE_ST_BLKSIZE
  + dlfcn.h

cmake/FindLibIntl.cmake

@@ -6,6 +6,7 @@
 #  LibIntl_LIBRARIES    - link these to use libintl
 
 include(CheckCSourceCompiles)
+include(CheckVariableExists)
 include(LibFindMacros)
 
 # Append custom gettext path to CMAKE_PREFIX_PATH
@@ -49,6 +50,10 @@ int main(int argc, char** argv) {
   textdomain(\"foo\");
 }" HAVE_WORKING_LIBINTL)
 
+if (HAVE_WORKING_LIBINTL)
+  check_variable_exists(_nl_msg_cat_cntr HAVE_NL_MSG_CAT_CNTR)
+endif()
+
 set(LibIntl_PROCESS_INCLUDES LibIntl_INCLUDE_DIR)
 set(LibIntl_PROCESS_LIBS LibIntl_LIBRARY)
 libfind_process(LibIntl)