* Use --with-gmp-incdir and --with-gmp-libdir (like for the main ecl
derivation), so ecl can figure out the flags to pass on its own.
Remove NIX_LDFLAGS hack for gmp.
* Make sure ecl can find boehmgc when linking programs / libraries via
NIX_LDFLAGS. Pass correct -I for boehmgc via NIX_CFLAGS_COMPILE since
there's no --with-boehmgc-prefix configure flag to achieve the same
result.
* Use correct suffixSalt for every flag. This makes NIX_CFLAGS_COMPILE
work properly in the first place and should prevent the extra flags
hacks leaking into other places in some edge cases.
