[ng] refactor: a first use of "ifndef VAR"

* lib/am/header-vars.am: Use "ifndef .FEATURES" instead of
"$(if $(.FEATURES))" to determine whether the used GNU make
version is recent enough.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>

diff --git a/lib/am/header-vars.am b/lib/am/header-vars.am
index 4ff47dba334a602cc52f280373f8e69e1d4ae122..9903af91a3d22c6fb30f90309e94654b3210377a 100644 (file)
--- a/lib/am/header-vars.am
+++ b/lib/am/header-vars.am
@@ -38,8 +38,9 @@ endef
 ## The .FEATURES special variable has been introduced in that make
 ## version, so use it as a witness to determine whether the current
 ## make is good enough.
-$(if $(.FEATURES),, \
-  $(error Automake-NG based builds require GNU make 3.81 or later))
+ifndef .FEATURES
+  $(error Automake-NG based builds require GNU make 3.81 or later)
+endif
 
 am__mkdir = test -d $1 || $(MKDIR_P) $1