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Compiling 1.6.1 is a different story though. I notice the configure script checks for curl_easy_init and after that does the pkg-config check for libcurl. Since it doesn't know where to look for curl yet, the former check obviously fails. Also, you use LIBCURL_CPPFLAGS, but on FreeBSD (and possibly other systems), pkg-config only returns LIBCURL_CFLAGS. Since they should be the same, could you change it to make use of CFLAGS instead of CPPFLAGS ?
libcurl.m4 defines LIBCURL_CPPFLAGS and CFLAGS would be something completely different as far as I understand it, so you're missing the m4 file from some package.
The configure script from the tarball (1.6.2) contains the following:
pkg_cv_LIBCURL_CFLAGS=`$PKG_CONFIG --cflags "libcurl >= 7.15.6" 2>/dev/null`
It doesn't set LIBCURL_CPPFLAGS anywhere (which makes pkg-config detection obsolete IMO). Also, in the Makefile.am it should be changed to LIBCURL_CFLAGS instead of LIBCURL_CPPFLAGS.