ac_std_float has some compiler warnings with clang-9

[lirp]

ac_std_float has some compiler warnings with clang-9.

For example, with -Wbitwise-conditional-parentheses and -Wunused-variable:
./ac_types/include/ac_std_float.h:429:23: error: unused variable 'rnd' [-Werror,-Wunused-variable]
static const bool rnd = QFX!=AC_TRN && QFX!=AC_TRN_ZERO;

...

./ac_types/include/ac_std_float.h:790:51: error: operator '?:' has lower precedence than '|'; '|' will be evaluated first [-Werror,-Wbitwise-conditional-parentheses]
int exp = ( (e1_lt_e2 & !op2_zero) | op1_zero ? op2_e_b : op1_e_b);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
./ac_types/include/ac_std_float.h:790:51: note: place parentheses around the '|' expression to silence this warning
int exp = ( (e1_lt_e2 & !op2_zero) | op1_zero ? op2_e_b : op1_e_b);
^
( )
./ac_types/include/ac_std_float.h:790:51: note: place parentheses around the '?:' expression to evaluate it first
int exp = ( (e1_lt_e2 & !op2_zero) | op1_zero ? op2_e_b : op1_e_b);

It would be really nice if this new header could be warnings-clean under clang, as disabling warnings transitively affects every translation unit that includes the header, and we've found these warnings to be particularly effective at keeping code bug-free.