componenet/aspell: force single thread compilation

this is a try at eliminating the following error during make install:

libtool: install: warning: relinking `email-filter.la'
libtool: install: (cd .../parts/aspell__compile__/aspell-0.60.7; /bin/bash .../parts/aspell__compile__/aspell-0.60.7/libtool  --tag CXX --mode=relink g++ -g -O2 -fno-exceptions -module -avoid-version -L.../parts/ncurses/lib -Wl,-rpath=.../parts/ncurses/lib -o email-filter.la -rpath .../parts/aspell/lib/aspell-0.60 modules/filter/email.lo libaspell.la -ldl -ldl )
libtool: relink: g++  -fPIC -DPIC -shared -nostdlib /usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/6/crtbeginS.o  modules/filter/.libs/email.o   -Wl,-rpath -Wl,.../parts/aspell/lib -L.../parts/ncurses/lib -L.../parts/aspell/lib -laspell -ldl -L/usr/lib/gcc/x86_64-linux-gnu/6 -L/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/6/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/6/../../.. -lstdc++ -lm -lc -lgcc_s /usr/lib/gcc/x86_64-linux-gnu/6/crtendS.o /usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/crtn.o  -O2 -Wl,-rpath=.../parts/ncurses/lib   -Wl,-soname -Wl,email-filter.so -o .libs/email-filter.so
/usr/bin/ld: cannot find -laspell

component/aspell/buildout.cfg

@@ -22,6 +22,8 @@ environment =
   PATH=${patch:location}/bin:${perl:location}/bin:%(PATH)s
   CPPFLAGS=-I${ncurses:location}/include
   LDFLAGS=-L${ncurses:location}/lib -Wl,-rpath=${ncurses:location}/lib
+make-options =
+  -j1
 
 [aspell-en-dictionary]
 <= aspell-dictionary-common