configure: split has_or_abort in two: one to check, one to abort

Split the has_or_abort function in two:
- one that checks if the tool if found,
- one that calls the above check, and aborts if not found

The rational behind this is to be able to check for a tool
and if not found, fallback to using our bundled version,
should the need arise (and I get time).

 # Compute AVR32-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

     # gcc ./configure flags

     CT_ARCH_WITH_ARCH=

     CT_ARCH_WITH_ABI=

     CT_ARCH_WITH_CPU=

     CT_ARCH_WITH_TUNE=

     CT_ARCH_WITH_FPU=

     CT_ARCH_WITH_FLOAT=

     CT_TARGET_SYS=none

 }