ExternalLib.mk: avoid pushd when cd is enough
ExternalLib.mk: avoid pushd when cd is enough

Reported-by: Julien Nabet <8a36cd9f353d245c534f8bb105741cdef632bfd5@yahoo.fr>
Acked-by: Norbert Thiebaud <4a068946920c47769d700757a47cfec2afcce997@gmail.com>

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core