net/openmpi: fix build on powerpc*

configure:16066: checking for short float
configure:16066: cc -c -DNDEBUG -O2 -pipe -I/usr/local/include/gcc13 -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -finline-functions -isystem /usr/local/include conftest.c >&5
conftest.c:149:13: error: 'short float' is invalid

149 | if (sizeof (short float))
    |             ^

1 error generated.