libdfp: Fix configure with compiler plus flags

This patch fix configure when issues defining a compiler as
path plus flags (for instance /my/compiler/gcc -m32). The downside
is configure with PATH to different toolchain will only works if the
same PATH is used when issuing make.
libdfp: Define DEC_NAN as __builtin_nan
PowerPC: Fix powerpc32 build

This patch fixes the missing hidden_def for __dpd_trunctdsd2 for
optimized PPC32 assembly.