contrib/fedora: explicitly set "pppd" path in configure script

We have "BuildRequires: ppp-devel". While in Fedora 37 that has a
dependency on "ppp" package, that is not the case on Centos7. I didn't
test others, but the point is it's not always there.

"/usr/sbin/pppd" is provided by "ppp" package, and we might not have it
installed via the build requirements. But we only need it to detect the
path, which is not necessary on RHEL/Fedora. Just set the path
explicitly with the respective configure option.

contrib/fedora/rpm/NetworkManager.spec

@@ -714,7 +714,8 @@ Preferably use nmcli instead.
 	-Difcfg_rh=true \
 	-Difupdown=false \
 %if %{with ppp}
-	-Dpppd_plugin_dir=%{_libdir}/pppd/%{ppp_version} \
+	-Dpppd_plugin_dir="%{_libdir}/pppd/%{ppp_version}" \
+	-Dpppd="%{_sbindir}/pppd" \
 	-Dppp=true \
 %endif
 %if %{with firewalld_zone}
@@ -855,8 +856,9 @@ autoreconf --install --force
 	--enable-ifcfg-rh=yes \
 	--enable-ifupdown=no \
 %if %{with ppp}
-	--with-pppd-plugin-dir=%{_libdir}/pppd/%{ppp_version} \
 	--enable-ppp=yes \
+	--with-pppd="%{_sbindir}/pppd" \
+	--with-pppd-plugin-dir="%{_libdir}/pppd/%{ppp_version}" \
 %endif
 %if %{with firewalld_zone}
 	--enable-firewalld-zone=yes \