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/basic/random-util.c

@@ -26,11 +26,11 @@
 #include <linux/random.h>
 #include <stdint.h>
 
-#ifdef HAVE_SYS_AUXV_H
+#if HAVE_SYS_AUXV_H
 #  include <sys/auxv.h>
 #endif
 
-#ifdef USE_SYS_RANDOM_H
+#if USE_SYS_RANDOM_H
 #  include <sys/random.h>
 #else
 #  include <linux/random.h>
@@ -100,14 +100,14 @@ int acquire_random_bytes(void *p, size_t n, bool high_quality_required) {
 void initialize_srand(void) {
         static bool srand_called = false;
         unsigned x;
-#ifdef HAVE_SYS_AUXV_H
+#if HAVE_SYS_AUXV_H
         void *auxv;
 #endif
 
         if (srand_called)
                 return;
 
-#ifdef HAVE_SYS_AUXV_H
+#if HAVE_SYS_AUXV_H
         /* The kernel provides us with 16 bytes of entropy in auxv, so let's
          * try to make use of that to seed the pseudo-random generator. It's
          * better than nothing... */