Support operating systems without some non-POSIX headers

The arpa/nameser.h, netinet/ip.h, and resolv.h headers are not crucial
to building LibreSSL. The netinet/ip.h header is used in nc(1) for
optional IPTOS_ features that can be ifdef'd on systems without support.

The endian.h header is the upcoming standard header and should be used
whenever available and correct. The machine/endian.h header is
non-standard and doesn't have to exist on POSIX systems.

Fix the check for getpagesize(3) not forward declaring the function,
such that CFLAGS with -Werror=implicit-function-declaration doesn't
cause the check to fail.

m4/check-libc.m4

@@ -1,6 +1,7 @@
 AC_DEFUN([CHECK_LIBC_COMPAT], [
 # Check for libc headers
 AC_CHECK_HEADERS([err.h readpassphrase.h])
+AC_CHECK_HEADERS([arpa/nameser.h endian.h netinet/ip.h resolv.h])
 # Check for general libc functions
 AC_CHECK_FUNCS([asprintf freezero memmem])
 AC_CHECK_FUNCS([readpassphrase reallocarray recallocarray])
@@ -9,10 +10,7 @@ AC_CHECK_FUNCS([timegm _mkgmtime timespecsub])
 AC_CHECK_FUNCS([getprogname syslog syslog_r])
 AC_CACHE_CHECK([for getpagesize], ac_cv_func_getpagesize, [
 	AC_LINK_IFELSE([AC_LANG_PROGRAM([[
-// Since Android NDK v16 getpagesize is defined as inline inside unistd.h
-#ifdef __ANDROID__
-#	include <unistd.h>
-#endif
+#include <unistd.h>
 		]], [[
 	getpagesize();
 ]])],