nall, desktop-ui: Account for other *nix data locations (#1688)

### Context

#1575 ended up defining the
`sharedData` directory on Unix-like systems as `~/.local/share` if a
build prefix was not defined. This is not exactly correct; the meaning
of a "shared data" directory on a Unix-like system is variously
`/usr/share`, `/usr/local/share`, or else the `share` directory of a
given install prefix.

### Changes

This PR allows us to respect normal Unix conventions while also
respecting the option to build and install with a given prefix. This is
done by defining two more functions in nall's `path`:
* `localSharedData()` for `/usr/local/share`
* `prefixSharedData()` for `${ARES_PREFIX}/share`

`sharedData()` is changed back to the old behavior and returns
`/usr/share`.

Then, the `locate` logic in desktop-ui (and uh, mia) is adjusted to now
search (in order):

1. the prefix's shared data if the prefix exists
2. local shared data
3. system shared data

desktop-ui/desktop-ui.cpp

@@ -19,6 +19,18 @@ auto locate(const string& name) -> string {
   if(inode::exists(location)) return location;
 
   // 3. The shared data directory
+#if defined(PLATFORM_LINUX) || defined(PLATFORM_BSD)
+  /// Unix-like systems have multiple notions of a 'shared data' directory. First, check for
+  /// an install prefix, as would be used by package managers that do not use `/usr/share`.
+  /// Secondly, look in `/usr/local/share` to cover software compiled by the user.
+  /// Lastly, look in the 'global' shared data directory, `/usr/share`.
+  location = {Path::prefixSharedData(), "ares/", name};
+  if(inode::exists(location)) return location;
+
+  location = {Path::localSharedData(), "ares/", name};
+  if(inode::exists(location)) return location;
+#endif
+
   location = {Path::sharedData(), "ares/", name};
   if(inode::exists(location)) return location;
 

mia/mia.cpp

@@ -21,6 +21,18 @@ auto locate(const string &name) -> string {
   if (inode::exists(location)) return location;
 
   // 3. The shared data directory
+#if defined(PLATFORM_LINUX) || defined(PLATFORM_BSD)
+  /// Unix-like systems have multiple notions of a 'shared data' directory. First, check for
+  /// an install prefix, as would be used by package managers that do not use `/usr/share`.
+  /// Secondly, look in `/usr/local/share` to cover software compiled by the user.
+  /// Lastly, look in the 'global' shared data directory, `/usr/share`.
+  location = {Path::prefixSharedData(), "ares/", name};
+  if (inode::exists(location)) return location;
+
+  location = {Path::localSharedData(), "ares/", name};
+  if (inode::exists(location)) return location;
+#endif
+
   location = {Path::sharedData(), "ares/", name};
   if (inode::exists(location)) return location;
 

nall/path.cpp

@@ -151,6 +151,23 @@ NALL_HEADER_INLINE auto sharedData() -> string {
   return result;
 }
 
+#if defined(PLATFORM_LINUX) || defined(PLATFORM_BSD)
+NALL_HEADER_INLINE auto prefixSharedData() -> string {
+  string result;
+  #if defined(ARES_PREFIX)
+  string result = stringize(ARES_PREFIX/share/);
+  #endif
+  if(!result) result = ".";
+  if(!result.endsWith("/")) result.append("/");
+  return result;
+}
+
+NALL_HEADER_INLINE auto localSharedData() -> string {
+  string result = "/usr/local/share/";
+  return result;
+}
+#endif
+
 NALL_HEADER_INLINE auto temporary() -> string {
   #if defined(PLATFORM_WINDOWS)
   wchar_t path[PATH_MAX] = L"";

nall/path.hpp

@@ -38,19 +38,19 @@ auto root() -> string;
 // c:/users/username/
 auto user() -> string;
 
-// /home/username/Desktop/
+// ~/Desktop/
 // c:/users/username/Desktop/
 auto desktop(string_view name = {}) -> string;
 
 //todo: MacOS uses the same location for userData() and userSettings()
 //... is there a better option here?
 
-// /home/username/.config/
+// ~/.config/
 // ~/Library/Application Support/
 // c:/users/username/appdata/roaming/
 auto userSettings() -> string;
 
-// /home/username/.local/share/
+// ~/.local/share/
 // ~/Library/Application Support/
 // c:/users/username/appdata/local/
 auto userData() -> string;
@@ -60,6 +60,15 @@ auto userData() -> string;
 // c:/ProgramData/
 auto sharedData() -> string;
 
+#if defined(PLATFORM_LINUX) || defined(PLATFORM_BSD)
+// ARES_PREFIX/share
+auto prefixSharedData() -> string;
+
+// /usr/local/share
+auto localSharedData() -> string;
+#endif
+
+
 // /tmp
 // c:/users/username/AppData/Local/Temp/
 auto temporary() -> string;