Avoid hardcoding the prefix in wx-config

When building flatpak modules, the prefix is /app and hardcoding /usr
no longer works.

wxGTK3.spec

@@ -87,8 +87,8 @@ ranging from networking to HTML display and image manipulation.
 Group:          Development/Libraries
 Summary:        Development files for the wxBase3 library
 Requires:       %{wxbasename}%{?_isa} = %{version}-%{release}
-Requires(post): %{_sbindir}/update-alternatives
-Requires(postun): %{_sbindir}/update-alternatives
+Requires(post): /usr/sbin/update-alternatives
+Requires(postun): /usr/sbin/update-alternatives
 
 %description -n %{wxbasename}-devel
 This package include files needed to link with the wxBase3 library.
@@ -354,6 +354,7 @@ popd
 rm %{buildroot}%{_bindir}/wx-config
 ##Install new and symlink
 install -p -D -m 755 %{SOURCE10} %{buildroot}%{_libexecdir}/%{name}/wx-config
+sed -i -e 's|=/usr|=%{_prefix}|' %{buildroot}%{_libexecdir}/%{name}/wx-config
 ln -s ../..%{_libexecdir}/%{name}/wx-config %{buildroot}%{_bindir}/wx-config-3.0
 touch %{buildroot}%{_bindir}/wx-config
 
@@ -388,15 +389,15 @@ popd
 if [ -f %{_bindir}/wx-config ] && [ ! -h %{_bindir}/wx-config ] ; then
   rm %{_bindir}/wx-config
 fi
-%{_sbindir}/update-alternatives --install %{_bindir}/wx-config \
+/usr/sbin/update-alternatives --install %{_bindir}/wx-config \
   wx-config %{_libexecdir}/%{name}/wx-config 3
-%{_sbindir}/update-alternatives --install %{_bindir}/wxrc \
+/usr/sbin/update-alternatives --install %{_bindir}/wxrc \
   wxrc %{_libexecdir}/%{name}/wxrc 3
 
 %postun -n %{wxbasename}-devel
 if [ $1 -eq 0 ] ; then
-  %{_sbindir}/update-alternatives --remove wx-config %{_libexecdir}/%{name}/wx-config
-  %{_sbindir}/update-alternatives --remove wxrc %{_libexecdir}/%{name}/wxrc
+  /usr/sbin/update-alternatives --remove wx-config %{_libexecdir}/%{name}/wx-config
+  /usr/sbin/update-alternatives --remove wxrc %{_libexecdir}/%{name}/wxrc
 fi
 
 %files