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/process-util.c

@@ -34,7 +34,7 @@
 #include <sys/wait.h>
 #include <syslog.h>
 #include <unistd.h>
-#ifdef HAVE_VALGRIND_VALGRIND_H
+#if HAVE_VALGRIND_VALGRIND_H
 #include <valgrind/valgrind.h>
 #endif
 
@@ -945,7 +945,7 @@ int opinionated_personality(unsigned long *ret) {
 }
 
 void valgrind_summary_hack(void) {
-#ifdef HAVE_VALGRIND_VALGRIND_H
+#if HAVE_VALGRIND_VALGRIND_H
         if (getpid_cached() == 1 && RUNNING_ON_VALGRIND) {
                 pid_t pid;
                 pid = raw_clone(SIGCHLD);