build: fix compilation on Cygwin (missing nanl/expl)
Formalize rarely used debug option

To build with debug option:
   ./configure --enable-debug
To use:
   compute --debug [other options]
maint: improve automatic checks script
build: remove unneeded header from src/compute.c

Compilation failed on Cygwin with this header.
build: work-around strtold() bug on cygwin
maint: add cygwin distribution packing script