build: re-accelerated the build

Simavr used to build pretty quick, and it's been crawling for a while
now, turns out some of the enhancements we added to the SIMAVR path
detection were being evaluated a LOT.
So went to the makefile and made sure most of the static variables are
evaluated once only.

Signed-off-by: Michel Pollet <buserror@gmail.com>
build: fix build of libsimavr.so

Was being linked with itself. Newer gcc are not amused.

Signed-off-by: Michel Pollet <buserror@gmail.com>
tests: Fix makefile

Was using ${OBJ} before it was declared.

Signed-off-by: Michel Pollet <buserror@gmail.com>