CMake: make it possible to use <PackageName>_ROOT to locate dependencies.

CMakeLists.txt

@@ -39,6 +39,11 @@ endif()
 if(POLICY CMP0071)
     cmake_policy(SET CMP0071 NEW)
 endif()
+# Allow <PackageName>_ROOT to be used on 3.12+ to point to per-package install
+# locations that find_package(PackageName) subsequently picks up
+if(POLICY CMP0074)
+    cmake_policy(SET CMP0074 NEW)
+endif()
 # Superprojects can use just set(MAGNUM_WITH_BLAH ON) without FORCE CACHE on
 # 3.13+
 if(POLICY CMP0077)

doc/building.dox

@@ -371,7 +371,9 @@ See @ref building-features "below" for additional configuration options.
 If you have the dependencies installed in non-standard location (other than
 `/usr`, e.g. `/home/xyz/projects`), set `CMAKE_PREFIX_PATH` to that directory
 to help CMake find them. You can enter more different dirs if you separate them
-with semicolons.
+with semicolons. On CMake 3.12+ you can also use `<PackageName>_ROOT` variables
+to point to dependency-specific locations, e.g.
+`-DCorrade_ROOT=/home/xyz/projects/corrade-install`.
 
 Also, if you plan to install the library to non-standard location, you might
 want to set `CMAKE_INSTALL_RPATH` to `lib/` subdir of given prefix (e.g.
@@ -392,7 +394,7 @@ want to set `CMAKE_INSTALL_RPATH` to `lib/` subdir of given prefix (e.g.
 
 On Windows you can use MSVC, clang-cl or the MinGW-w64 compiler. It's then up
 to you whether you will use QtCreator, Visual Studio or another IDE or do the
-build from a command line. Note that for most convenient usage it's best use
+build from a command line. Note that for most convenient usage it's best to use
 some dedicated directory (e.g. `C:/Sys`) for installing dependencies instead of
 putting each dependency to its own directory in `C:/Program Files` or
 elsewhere. Then you can just add its `bin/` subdir (e.g. `C:/Sys/bin`) to
@@ -403,7 +405,9 @@ prepared for exactly this.
 Then, when running CMake, set `CMAKE_PREFIX_PATH` and `CMAKE_INSTALL_PREFIX`
 value to that directory (e.g. `-DCMAKE_INSTALL_PREFIX=C:/Sys`). If you have
 dependencies scattered across multiple locations, enter them into
-`CMAKE_PREFIX_PATH` separated with semicolons.
+`CMAKE_PREFIX_PATH` separated with semicolons. On CMake 3.12+ you can also
+use `<PackageName>_ROOT` variables to point to dependency-specific locations,
+e.g. `-DCorrade_ROOT=C:/projects/corrade-install`.
 
 @subsubsection building-windows-msvc Using Visual Studio
 

doc/changelog.dox

@@ -733,6 +733,9 @@ See also:
 -   @ref Trade::ObjImporter "ObjImporter" uses exceptions internally and needs
     an explicit exception-enabling flag when built with Emscripten 1.39.0 and
     newer
+-   It's now possible to use `<PackageName>_ROOT` to point to install locations
+    of dependencies such as Corrade on CMake 3.12+, in addition to putting them
+    all together inside `CMAKE_PREFIX_PATH`. See also [mosra/magnum#614](https://github.com/mosra/magnum/issues/614).
 -   On CMake 3.16 and newer, `FindMagnum.cmake` can provide additional details
     if some component is not found.
 -   The Homebrew package now uses `std_cmake_args` instead of hardcoded build