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

docs/sysvinit/meson.build

@@ -3,7 +3,7 @@ file = configure_file(
         output : 'README',
         configuration : substs)
 
-if conf.get('HAVE_SYSV_COMPAT', false)
+if conf.get('HAVE_SYSV_COMPAT') == 1
         install_data(file,
                      install_dir : sysvinit_path)
 endif