Add -pthread generally

Not sure if deduplicated or otherwise lost, but the link order
in ADIOS2 seems to be
```
... libadios2_taustubs.a -ldl -lpthread /usr/local/lib/libblosc.a ...
```

library_builders.sh

@@ -196,6 +196,10 @@ function build_hdf5 {
 # static libs need relocatable symbols for linking to shared python lib
 export CFLAGS+=" -fPIC"
 export CXXFLAGS+=" -fPIC"
+# Blosc PThread propagation does not work well at least on ppc64le
+export CFLAGS+=" -pthread"
+export CXXFLAGS+=" -pthread"
+export LDFLAGS+=" -pthread"
 
 install_buildessentials
 build_blosc