SCons: Remove check_c_headers

• Can instead check for headers directly with `__has_include`, a C++17 feature

drivers/unix/SCsub

@@ -4,5 +4,3 @@ from misc.utility.scons_hints import *
 Import("env")
 
 env.add_source_files(env.drivers_sources, "*.cpp")
-
-env["check_c_headers"] = {"mntent.h": "HAVE_MNTENT"}

drivers/unix/dir_access_unix.cpp

@@ -46,7 +46,7 @@
 #include <sys/stat.h>
 #include <sys/statvfs.h>
 
-#ifdef HAVE_MNTENT
+#if __has_include(<mntent.h>)
 #include <mntent.h>
 #endif
 
@@ -192,7 +192,7 @@ void DirAccessUnix::list_dir_end() {
 	_cisdir = false;
 }
 
-#if defined(HAVE_MNTENT) && defined(LINUXBSD_ENABLED)
+#if __has_include(<mntent.h>) && defined(LINUXBSD_ENABLED)
 static bool _filter_drive(struct mntent *mnt) {
 	// Ignore devices that don't point to /dev
 	if (strncmp(mnt->mnt_fsname, "/dev", 4) != 0) {
@@ -216,7 +216,7 @@ static void _get_drives(List<String> *list) {
 	// Add root.
 	list->push_back("/");
 
-#if defined(HAVE_MNTENT) && defined(LINUXBSD_ENABLED)
+#if __has_include(<mntent.h>) && defined(LINUXBSD_ENABLED)
 	// Check /etc/mtab for the list of mounted partitions.
 	FILE *mtab = setmntent("/etc/mtab", "r");
 	if (mtab) {