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/path-util.h

@@ -30,7 +30,7 @@
 #define DEFAULT_PATH_NORMAL "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin"
 #define DEFAULT_PATH_SPLIT_USR DEFAULT_PATH_NORMAL ":/sbin:/bin"
 
-#ifdef HAVE_SPLIT_USR
+#if HAVE_SPLIT_USR
 #  define DEFAULT_PATH DEFAULT_PATH_SPLIT_USR
 #else
 #  define DEFAULT_PATH DEFAULT_PATH_NORMAL