The nds32 & spu dirs are using compile tests to look for some
preprocessor defines, but we don't need to compile the code,
just preprocess it. So switch to AC_PREPROC_IFELSE.
The sh dir is using a preprocessor test via grep, but let's
switch it to AC_PREPROC_IFELSE too to be consistent.
This should allow us to drop the uncommon AC_NO_EXECUTABLES call.
2e9aa5f56c