build: fix Descent3 program link failure

In case httplibConfig.cmake is absent, there is still a link failure
because -lcpp-httplib appears nowhere on the command-line.

```
/usr/include/c++/14/bits/unique_ptr.h:1077:(.text+0x7ae9): undefined reference to `httplib::Client::Client(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/lib64/gcc/x86_64-suse-linux/14/../../../../x86_64-suse-linux/bin/ld: /usr/include/c++/14/bits/unique_ptr.h:93:(.text+0x7b0b): undefined reference to `httplib::Client::~Client()'
```

The commit message of 88e487 was a dead giveaway:

	>"only find the header"

A header is not enough to get the build job done.

Fixes: 88e487c

Author: jengelh

CMakeLists.txt

@@ -189,6 +189,7 @@ else()
 
   # The target httplib::httplib cannot be created manually, so rename it
   add_library(httplib INTERFACE ${CPP_HTTPLIB_HEADER})
+  target_link_libraries(httplib INTERFACE -lcpp-httplib)
 endif()
 find_package(ZLIB REQUIRED)
 find_package(plog REQUIRED)