openblas: make clean is required for fallback build case (cf nexedi/slapos!42).

component/openblas/buildout.cfg

@@ -23,7 +23,7 @@ build-ext-options =
 
 # First try with auto-detected target and if it fails try TARGET=GENERIC.
 configure-command =
-  make ${:build-common-options} ${:build-ext-options} || make ${:build-common-options} TARGET=GENERIC
+  make ${:build-common-options} ${:build-ext-options} || (make -j1 clean && make ${:build-common-options} TARGET=GENERIC)
 make-options =
   dummy
 make-targets =