Set CMAKE_INSTALL_PREFIX default more correctly

cmake/compilers/gnu.cmake

@@ -25,7 +25,3 @@ add_compile_options(-fno-strict-aliasing)
 # This is necessary to hide all symbols unless explicitly exported
 # via the Q_EXPORT macro
 add_compile_options(-fvisibility=hidden)
-
-if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
-    set(CMAKE_INSTALL_PREFIX /opt/quake3)
-endif()

cmake/platforms/all.cmake

@@ -1,4 +1,5 @@
 include(platforms/emscripten)
+include(platforms/linux)
 include(platforms/macos)
 include(platforms/unix)
 include(platforms/windows)

cmake/platforms/linux.cmake

@@ -0,0 +1,9 @@
+# Linux-like specific settings, i.e. including FreeBSD etc.
+
+if(NOT UNIX OR APPLE)
+    return()
+endif()
+
+if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
+    set_property(CACHE CMAKE_INSTALL_PREFIX PROPERTY VALUE /opt/quake3)
+endif()