build-sys: fix type detection

Before this patch existence of char16_t, char32_t, key_serial_t was checked
with AC_CHECK_DECLS() which doesn't actually work for types. Correct this to
use AC_CHECK_TYPES() instead.

Also, while we are at it, change the check for memfd_create() to use
AC_CHECK_DECLS() instead of AC_CHECK_FUNCS(). This is a better choice, since a
couple of syscalls are defined by glibc but not exported in the header files
(pivot_root() for example), and we hence should probably be more picky with
memfd_create() too, which glibc might decide to expose one day, but not
necessarily in the headers too.

src/basic/missing.h

@@ -167,7 +167,7 @@ static inline int pivot_root(const char *new_root, const char *put_old) {
 #  endif
 #endif
 
-#ifndef HAVE_MEMFD_CREATE
+#if !HAVE_DECL_MEMFD_CREATE
 static inline int memfd_create(const char *name, unsigned int flags) {
         return syscall(__NR_memfd_create, name, flags);
 }
@@ -1089,7 +1089,7 @@ static inline int kcmp(pid_t pid1, pid_t pid2, int type, unsigned long idx1, uns
 #define INPUT_PROP_ACCELEROMETER  0x06
 #endif
 
-#if !HAVE_DECL_KEY_SERIAL_T
+#ifndef HAVE_KEY_SERIAL_T
 typedef int32_t key_serial_t;
 #endif
 
@@ -1160,11 +1160,11 @@ static inline key_serial_t request_key(const char *type, const char *description
 #ifndef IF_OPER_UP
 #define IF_OPER_UP 6
 
-#ifndef HAVE_DECL_CHAR32_T
+#ifndef HAVE_CHAR32_T
 #define char32_t uint32_t
 #endif
 
-#ifndef HAVE_DECL_CHAR16_T
+#ifndef HAVE_CHAR16_T
 #define char16_t uint16_t
 #endif