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/sysusers/sysusers.c

@@ -299,7 +299,7 @@ static int putgrent_with_members(const struct group *gr, FILE *group) {
         return 0;
 }
 
-#ifdef ENABLE_GSHADOW
+#if ENABLE_GSHADOW
 static int putsgent_with_members(const struct sgrp *sg, FILE *gshadow) {
         char **a;
 
@@ -636,7 +636,7 @@ static int write_temporary_group(const char *group_path, FILE **tmpfile, char **
 }
 
 static int write_temporary_gshadow(const char * gshadow_path, FILE **tmpfile, char **tmpfile_path) {
-#ifdef ENABLE_GSHADOW
+#if ENABLE_GSHADOW
         _cleanup_fclose_ FILE *original = NULL, *gshadow = NULL;
         _cleanup_(unlink_and_freep) char *gshadow_tmp = NULL;
         bool group_changed = false;