make extra_options empty in the correct way

armadillo.spec

@@ -1,6 +1,6 @@
 Name:           armadillo
 Version:        9.900.3
-Release:        1%{?dist}
+Release:        2%{?dist}
 Summary:        Fast C++ matrix library with syntax similar to MATLAB and Octave
 
 License:        ASL 2.0
@@ -15,7 +15,7 @@ Source:         http://sourceforge.net/projects/arma/files/%{name}-%{version}.ta
 %global extra_options -DALLOW_FLEXIBLAS_LINUX=ON
 BuildRequires:  flexiblas-devel
 %else
-%global extra_options
+%global extra_options %{nil}
 %endif
 
 BuildRequires:  gcc-c++
@@ -114,6 +114,9 @@ make -C "%{_vpath_builddir}"
 
 
 %changelog
+* Fri Sep  4 2020 José Matos <jamatos@fedoraproject.org> - 9.900.3-2
+- make extra_options empty in the correct way
+
 * Thu Sep  3 2020 José Matos <jamatos@fedoraproject.org> - 9.900.3-1
 - update to 9.900.3