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()'
```

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

But… it turns out cpp-httplib can either be built as a classic
header + library combo, or installed in a header-only fashion. *sigh*
Deal with it.

Fixes: 88e487c724

CMakeLists.txt

@@ -186,9 +186,16 @@ if (TARGET httplib::httplib)
 else()
   # Some Linux distributions do not package httplibConfig.cmake, so find the header manually
   find_file(CPP_HTTPLIB_HEADER httplib.h REQUIRED)
+  find_library(CPP_HTTPLIB_LIB cpp-httplib QUIET)
 
   # The target httplib::httplib cannot be created manually, so rename it
   add_library(httplib INTERFACE ${CPP_HTTPLIB_HEADER})
+
+  # And some Linux distributions build httplib as header-only,
+  # and some as header+library. What a mess!
+  if (NOT ${CPP_HTTPLIB_LIB} STREQUAL "CPP_HTTPLIB_LIB-NOTFOUND")
+    target_link_libraries(httplib INTERFACE ${CPP_HTTPLIB_LIB})
+  endif ()
 endif()
 find_package(ZLIB REQUIRED)
 find_package(plog REQUIRED)