treewide: only define feature macros when the feature is available

Most feature checks already use #ifdef, and do not care about
the value of the macro. Convert all feature checks to do that,
and simplify the meson build scripts by replacing

  if cond
    cdata.set('X', 1)
  endif

with

  cdata.set('X', cond)

test/pwtest-compat.c

@@ -24,7 +24,7 @@
 
 #include "config.h"
 
-#if !HAVE_SIGABBREV_NP
+#ifndef HAVE_SIGABBREV_NP
 #include <stddef.h>
 #include <signal.h>
 

test/pwtest.c

@@ -36,10 +36,10 @@
 #include <stdio.h>
 #include <unistd.h>
 #include <signal.h>
-#if HAVE_PIDFD_OPEN
+#ifdef HAVE_PIDFD_OPEN
 #include <sys/syscall.h>
 #endif
-#if HAVE_LIBCAP
+#ifdef HAVE_LIBCAP
 #include <sys/capability.h>
 #endif
 #include <sys/epoll.h>
@@ -177,7 +177,7 @@ static void restore_env(struct pwtest_test *t)
 
 static void pwtest_backtrace(pid_t p)
 {
-#if HAVE_GSTACK
+#ifdef HAVE_GSTACK
 	char pid[11];
 	pid_t parent, child;
 	int status;
@@ -820,7 +820,7 @@ static int monitor_test_forked(struct pwtest_test *t, pid_t pid, int read_fds[_F
 	size_t nevents = 0;
 	int r;
 
-#if HAVE_PIDFD_OPEN
+#ifdef HAVE_PIDFD_OPEN
 	pidfd = syscall(SYS_pidfd_open, pid, 0);
 #else
 	errno = ENOSYS;
@@ -1168,7 +1168,7 @@ static void list_tests(struct pwtest_context *ctx)
 static bool is_debugger_attached(void)
 {
 	bool rc = false;
-#if HAVE_LIBCAP
+#ifdef HAVE_LIBCAP
 	int status;
 	int pid = fork();
 

test/test-context.c

@@ -233,7 +233,7 @@ PWTEST(context_support)
 		SPA_TYPE_INTERFACE_Loop,
 		SPA_TYPE_INTERFACE_LoopUtils,
 		SPA_TYPE_INTERFACE_Log,
-#if HAVE_DBUS
+#ifdef HAVE_DBUS
 		SPA_TYPE_INTERFACE_DBus,
 #endif
 		SPA_TYPE_INTERFACE_CPU

test/test-pwtest.c

@@ -32,7 +32,7 @@
 
 PWTEST(compat_sigabbrev_np)
 {
-#if !HAVE_SIGABBREV_NP
+#ifndef HAVE_SIGABBREV_NP
 	pwtest_str_eq(sigabbrev_np(SIGABRT), "ABRT");
 	pwtest_str_eq(sigabbrev_np(SIGSEGV), "SEGV");
 	pwtest_str_eq(sigabbrev_np(SIGSTOP), "STOP");