Git configure with static curl is failing

[Paul Smith <paul@mad-scientist.net>]

Hi all;

I'm trying to build Git with my own static curl, not the system curl,
and this isn't working due to an issue in configure.ac's curl locating
code.

I invoke configure with --with-curl=/other/curl where
/other/curl/include/curl and /other/curl/lib/libcurl.a and
/other/curl/bin/curl-config all exist and work correctly.

When configure.ac attempts to local curl, it fails; config.log shows:

  configure:5387: my-gcc -o conftest -O2 \
   -I/other/curl/include -L/other/curl/lib conftest.c -lcurl >&5
  /other/curl/lib/libcurl.a(libcurl_la-content_encoding.o):content_encoding.c:function gzip_init_writer: error: undefined reference to 'zlibVersion'

and many other errors, because the link line is missing needed
libraries.  If curl as a .so this wouldn't matter since libcurl.so
would contain references to the libraries it needs and ld.so would
handle this, but a static library obviously doesn't have that.

The problem appears to be that configure.ac is not using curl-config to
obtain the correct options to link with curl.  If I run
/other/curl/bin/curl-config --lib I get the proper flags:

  $ /other/curl/bin/curl-config --libs
  -L/other/curl/lib -lcurl -lssl -lz -lcrypto -ldl -lpthread

If configure.ac tried to use $CURLDIR/bin/curl-config --libs to obtain
the right linker options, then the link test for curl would work.