Revert %_smp_build_ncpus change to a parametric macro (RhBug:2210347)

Commit a213101 changed %_smp_build_ncpus
into a parametric macro, but this breaks common usage via the Lua macros
table as parametric macros are returned as closures rather than the
expanded value.

This seems like a design flaw of the macros table, but as an immediate
remedy for the breakage, add another layer of indirection to revert
%_smp_build_ncpus back to a non-parametric macro.

Fixes %constrain_build macro in Fedora, which ironically is made obsolete by
the change that (unintentionally) broke it.

macros.in

@@ -717,21 +717,23 @@ Supplements:   (%{name} = %{version}-%{release} and langpacks-%{1})\
 # Macro to fix broken permissions in sources
 %_fixperms      %{__chmod} -Rf a+rX,u+w,g-w,o-w
 
-# Maximum number of CPU's to use when building, 0 for unlimited.
-#%_smp_ncpus_max 0
-
-%_smp_build_ncpus() %([ -z "$RPM_BUILD_NCPUS" ] \\\
+%__smp_use_ncpus() %([ -z "$RPM_BUILD_NCPUS" ] \\\
 	&& RPM_BUILD_NCPUS="%{getncpus %{?1}}"; \\\
         ncpus_max=%{?_smp_ncpus_max}; \\\
         if [ -n "$ncpus_max" ] && [ "$ncpus_max" -gt 0 ] && [ "$RPM_BUILD_NCPUS" -gt "$ncpus_max" ]; then RPM_BUILD_NCPUS="$ncpus_max"; fi; \\\
         echo "$RPM_BUILD_NCPUS";)
 
+# Maximum number of CPU's to use when building, 0 for unlimited.
+#%_smp_ncpus_max 0
+
+%_smp_build_ncpus %{__smp_use_ncpus:proc}
+
 %_smp_mflags -j${RPM_BUILD_NCPUS}
 
 # Maximum number of threads to use when building, 0 for unlimited
 #%_smp_nthreads_max 0
 
-%_smp_build_nthreads %{_smp_build_ncpus:thread}
+%_smp_build_nthreads %{__smp_use_ncpus:thread}
 
 # Assumed task size of processes and threads in megabytes.
 # Used to limit the amount of parallelism based on available memory.