run the gunlib checks with same compiler as when building

The gnulib will be built with $CC and not $CXX so we should not run the
gnulib tests with $CXX.

This fixes lavv17/lftp#59
check if we need set __STDC_LIMIT_MACROS before including stdint.h

Older glibc and uclibc requires that __STDC_LIMIT_MACROS is defined
before use of stdint.h in C++ programs.

This should probably be handeled better in gnulib, but rather than
try fix gnulib we simply workaround it by checking if
__STDC_LIMIT_MACROS is needed and if so, define it in config.h
Merge pull request #60 from ncopa/master

run the gunlib checks with same compiler as when building