Add workaround for el7 (does not work yet due to freerdp being too old)

remmina.spec

@@ -245,8 +245,14 @@ sed -i -e '/x-scheme-handler/d' data/desktop/remmina-file.desktop.in
 %build
 mkdir -p build
 
+%if 0%{?fedora}
 # Workaround for Pango on Fedora 31+
 export CFLAGS="%{optflags} -I%{_includedir}/harfbuzz"
+%endif
+
+%if 0%{?rhel} == 7
+export CFLAGS="%{optflags} -std=gnu99"
+%endif
 
 %if 0%{?fedora} || 0%{?rhel} >= 8
 %cmake --build=build \