build-sys: use #if Y instead of #ifdef Y everywhere

The advantage is that is the name is mispellt, cpp will warn us.

$ git grep -Ee "conf.set\('(HAVE|ENABLE)_" -l|xargs sed -r -i "s/conf.set\('(HAVE|ENABLE)_/conf.set10('\1_/"
$ git grep -Ee '#ifn?def (HAVE|ENABLE)' -l|xargs sed -r -i 's/#ifdef (HAVE|ENABLE)/#if \1/; s/#ifndef (HAVE|ENABLE)/#if ! \1/;'
$ git grep -Ee 'if.*defined\(HAVE' -l|xargs sed -i -r 's/defined\((HAVE_[A-Z0-9_]*)\)/\1/g'
$ git grep -Ee 'if.*defined\(ENABLE' -l|xargs sed -i -r 's/defined\((ENABLE_[A-Z0-9_]*)\)/\1/g'
+ manual changes to meson.build

squash! build-sys: use #if Y instead of #ifdef Y everywhere

v2:
- fix incorrect setting of HAVE_LIBIDN2

src/nspawn/nspawn-patch-uid.c

@@ -19,7 +19,7 @@
 
 #include <fcntl.h>
 #include <linux/magic.h>
-#ifdef HAVE_ACL
+#if HAVE_ACL
 #include <sys/acl.h>
 #endif
 #include <sys/stat.h>
@@ -37,7 +37,7 @@
 #include "strv.h"
 #include "user-util.h"
 
-#ifdef HAVE_ACL
+#if HAVE_ACL
 
 static int get_acl(int fd, const char *name, acl_type_t type, acl_t *ret) {
         char procfs_path[strlen("/proc/self/fd/") + DECIMAL_STR_MAX(int) + 1];