make and parallel builds.

There is a problem that parallel builds do not execute successfully every time.

We are building many static libraries in parallel (make -j8), using GNU
make, on makefiles which are generated with qmake. It happens quite often that
make
fails in the last step:
ranlib -s libXXXX.a
ranlib: for architecture: i386 file: libXXXX.a(XXXX.o) has no symbols
ranlib: for architecture: ppc file: libXXXX.a(XXXX.o) has no symbols
ranlib: fat file: libXXXX.a truncated or malformed ([...])
ar: internal ranlib command failed
make[1]: *** [/Volumes/debug/libXXXX.a] Error 1
make: *** [debug-all] Error 2
make: *** Waiting for unfinished jobs....

This make reliable builds problematic.