Use CMAKE_INSTALL_SYSCONFDIR instead of /etc (#2770)

Some distro's build infrastructure (e.g. gentoo) can use
CMAKE_INSTALL_PREFIX to install to a different location.
When the config path is hardcoded to /etc, CMAKE_INSTALL_PREFIX has no
effect.

Ref: https://bugs.gentoo.org/858797

CHANGELOG.md

@@ -12,6 +12,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Breaking
 - `custom/script`: now doesn't hide failing script if it's output is not changing ([`#2636`](https://github.com/polybar/polybar/issues/2636)). Somewhat similar behaviour can be imitated with `format-fail`, if necessary.
 
+### Build
+- Respect `CMAKE_INSTALL_PREFIX` when installing default config ([`#2770`](https://github.com/polybar/polybar/pull/2770))
+
 ### Deprecated
 - `custom/text`: The `content` setting and all its properties are deprecated in favor of `format` with the same functionality. ([`#2676`](https://github.com/polybar/polybar/pull/2676))
 

CMakeLists.txt

@@ -63,7 +63,7 @@ endif()
 
 if(BUILD_CONFIG)
   install(FILES ${CMAKE_SOURCE_DIR}/doc/config.ini
-    DESTINATION /etc/${PROJECT_NAME}
+    DESTINATION ${CMAKE_INSTALL_SYSCONFDIR}/${PROJECT_NAME}
     COMPONENT config)
 endif()