The link failure happens on a system with stable nix-2.3.15
installed in /usr/lib64 (it's libutil.so API differs from master):
```
LANG=C make V=1
g++ -o /home/slyfox/dev/git/nix/src/libstore/libnixstore.so \
-shared -L/usr/lib64 -Wl,--no-copy-dt-needed-entries \
src/libstore/binary-cache-store.o ... src/libstore/uds-remote-store.o \
-lsqlite3 -lcurl -lsodium -pthread -ldl -lseccomp -Wl,-z,defs -Wl,-soname=libnixstore.so
-Wl,-rpath,/home/slyfox/dev/git/nix/src/libutil -Lsrc/libutil -lnixutil
ld: src/libstore/binary-cache-store.o: in function `nix::BinaryCacheStore::BinaryCacheStore(
std::map<std::__cxx11::basic_string<char, std::char_traits<char>, ...
nix/src/libstore/binary-cache-store.cc:30: undefined reference to `nix::readFile(
std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)' ...
...
```
This happens due to `-L/usr/lib64 -Lsrc/libutil` search path ordering.
The change turns it into `-Lsrc/libutil -L/usr/lib64`.
Closes: https://github.com/NixOS/nix/issues/3087
autotools-based systems usually allow user to
append own LDFLAGS like
LDFLAGS="-Wl,-O1 -Wl,--as-needed -Wl,--hash-style=gnu"
at ./configure stage
This change plumbs LDFLAGS through similar to existing CXXFLAGS variable.
Signed-off-by: Sergei Trofimovich <siarheit@google.com>
