Fix -Wundef warnings for patterns `#if HAVE`

* See [Feature #17752]
* Using this to detect them:
  git grep -P 'if\s+HAVE' | grep -Pv 'HAVE_LONG_LONG|/ChangeLog|HAVE_TYPEOF'

dir.c

@@ -50,13 +50,13 @@
 # define dirent direct
 # define NAMLEN(dirent) (dirent)->d_namlen
 # define HAVE_DIRENT_NAMLEN 1
-# if HAVE_SYS_NDIR_H
+# ifdef HAVE_SYS_NDIR_H
 #  include <sys/ndir.h>
 # endif
-# if HAVE_SYS_DIR_H
+# ifdef HAVE_SYS_DIR_H
 #  include <sys/dir.h>
 # endif
-# if HAVE_NDIR_H
+# ifdef HAVE_NDIR_H
 #  include <ndir.h>
 # endif
 # ifdef _WIN32

ext/bigdecimal/missing.h

@@ -45,7 +45,7 @@ extern "C" {
 # if __has_builtin(__builtin_unreachable)
 #  define UNREACHABLE __builtin_unreachable()
 
-# elif HAVE___ASSUME
+# elif defined(HAVE___ASSUME)
 #  define UNREACHABLE __assume(0)
 
 # else

ext/psych/yaml/yaml_private.h

@@ -2,7 +2,7 @@
 #include RUBY_EXTCONF_H
 #endif
 
-#if HAVE_CONFIG_H
+#ifdef HAVE_CONFIG_H
 #include "config.h"
 #endif
 

gc.c

@@ -45,7 +45,7 @@
 #ifdef HAVE_MALLOC_USABLE_SIZE
 # ifdef RUBY_ALTERNATIVE_MALLOC_HEADER
 #  include RUBY_ALTERNATIVE_MALLOC_HEADER
-# elif HAVE_MALLOC_H
+# elif defined(HAVE_MALLOC_H)
 #  include <malloc.h>
 # elif defined(HAVE_MALLOC_NP_H)
 #  include <malloc_np.h>
@@ -381,7 +381,7 @@ static ruby_gc_params_t gc_params = {
 #endif
 #if RGENGC_DEBUG < 0 && !defined(_MSC_VER)
 # define RGENGC_DEBUG_ENABLED(level) (-(RGENGC_DEBUG) >= (level) && ruby_rgengc_debug >= (level))
-#elif HAVE_VA_ARGS_MACRO
+#elif defined(HAVE_VA_ARGS_MACRO)
 # define RGENGC_DEBUG_ENABLED(level) ((RGENGC_DEBUG) >= (level))
 #else
 # define RGENGC_DEBUG_ENABLED(level) 0

random.c

@@ -428,7 +428,7 @@ random_init(int argc, VALUE *argv, VALUE obj)
 # define USE_DEV_URANDOM 0
 #endif
 
-#if HAVE_GETENTROPY
+#ifdef HAVE_GETENTROPY
 # define MAX_SEED_LEN_PER_READ 256
 static int
 fill_random_bytes_urandom(void *seed, size_t size)

signal.c

@@ -29,7 +29,7 @@
 # include <ucontext.h>
 #endif
 
-#if HAVE_PTHREAD_H
+#ifdef HAVE_PTHREAD_H
 # include <pthread.h>
 #endif
 

siphash.c

@@ -180,7 +180,7 @@ int_sip_dump(sip_state *state)
     int v;
 
     for (v = 0; v < 4; v++) {
-#if HAVE_UINT64_T
+#ifdef HAVE_UINT64_T
 	printf("v%d: %" PRIx64 "\n", v, state->v[v]);
 #else
 	printf("v%d: %" PRIx32 "%.8" PRIx32 "\n", v, state->v[v].hi, state->v[v].lo);
@@ -447,7 +447,7 @@ sip_hash13(const uint8_t key[16], const uint8_t *data, size_t len)
 	    OR_BYTE(4);
 	case 4:
 #if BYTE_ORDER == LITTLE_ENDIAN && UNALIGNED_WORD_ACCESS
-  #if HAVE_UINT64_T
+  #ifdef HAVE_UINT64_T
 	    last |= (uint64_t) ((uint32_t *) end)[0];
   #else
 	    last.lo |= ((uint32_t *) end)[0];

strftime.c

@@ -59,7 +59,7 @@
 #endif
 #if defined(TM_IN_SYS_TIME) || !defined(GAWK)
 #include <sys/types.h>
-#if HAVE_SYS_TIME_H
+#ifdef HAVE_SYS_TIME_H
 #include <sys/time.h>
 #endif
 #endif

thread_pthread.c

@@ -20,9 +20,9 @@
 #ifdef HAVE_THR_STKSEGMENT
 #include <thread.h>
 #endif
-#if HAVE_FCNTL_H
+#if defined(HAVE_FCNTL_H)
 #include <fcntl.h>
-#elif HAVE_SYS_FCNTL_H
+#elif defined(HAVE_SYS_FCNTL_H)
 #include <sys/fcntl.h>
 #endif
 #ifdef HAVE_SYS_PRCTL_H