fixup! build: Generate share/toolchain.cmake in depends

Define depends prefix relatively to the toolchain file location (the
same way it is done for the `config.site` file).

Should fix building on NixOS.

depends/Makefile

@@ -272,7 +272,6 @@ $(host_prefix)/share/toolchain.cmake : toolchain.cmake.in $(host_prefix)/.stamp_
             -e 's|@OBJCOPY@|$(host_OBJCOPY)|' \
             -e 's|@INSTALL_NAME_TOOL@|$(host_INSTALL_NAME_TOOL)|' \
             -e 's|@OTOOL@|$(host_OTOOL)|' \
-            -e 's|@depends_prefix@|$(host_prefix)|' \
             -e 's|@CFLAGS@|$(strip $(host_CFLAGS) $(host_$(release_type)_CFLAGS))|' \
             -e 's|@CXXFLAGS@|$(strip $(host_CXXFLAGS) $(host_$(release_type)_CXXFLAGS))|' \
             -e 's|@CPPFLAGS@|$(strip $(host_CPPFLAGS) $(host_$(release_type)_CPPFLAGS))|' \

depends/toolchain.cmake.in

@@ -59,18 +59,31 @@ set(CMAKE_OBJCOPY "@OBJCOPY@")
 set(CMAKE_INSTALL_NAME_TOOL "@INSTALL_NAME_TOOL@")
 set(OTOOL "@OTOOL@")
 
-# Using our own built dependencies should not be
-# affected by a potentially random environment.
-set(CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH OFF)
 
-set(CMAKE_FIND_ROOT_PATH "@depends_prefix@")
+# Path settings
+# =============
+
+# Point CMake to search in the ${CMAKE_FIND_ROOT_PATH}/${CMAKE_PREFIX_PATH}
+# directory when using find_library(), find_file(), find_path(), and
+# find_package() commands.
+cmake_path(GET CMAKE_CURRENT_LIST_DIR PARENT_PATH CMAKE_FIND_ROOT_PATH)
+cmake_path(SET CMAKE_PREFIX_PATH "/")
 set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
 set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
-set(PKG_CONFIG_PATH "@depends_prefix@/lib/pkgconfig")
+# Using our own built dependencies should not be affected by a potentially
+# random environment.
+set(CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH NO)
+set(CMAKE_FIND_USE_CMAKE_SYSTEM_PATH NO)
+
+cmake_path(APPEND CMAKE_FIND_ROOT_PATH "lib" "pkgconfig" OUTPUT_VARIABLE PKG_CONFIG_PATH)
 set(PKG_CONFIG_LIBDIR "${PKG_CONFIG_PATH}")
-set(QT_TRANSLATIONS_DIR "@depends_prefix@/translations")
+# Defining the prefix variable explicitly makes depends relocatable, which
+# simplifies, for example, building on NixOS.
+set(PKG_CONFIG_ARGN "--define-variable=prefix=${CMAKE_FIND_ROOT_PATH}")
+
+cmake_path(APPEND CMAKE_FIND_ROOT_PATH "translations" OUTPUT_VARIABLE QT_TRANSLATIONS_DIR)
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND NOT CMAKE_HOST_APPLE)
   # The find_package(Qt ...) function internally uses find_library()
@@ -80,6 +93,10 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND NOT CMAKE_HOST_APPLE)
   set(CMAKE_FRAMEWORK_PATH "@OSX_SDK@/System/Library/Frameworks")
 endif()
 
+
+# Build options
+# =============
+
 # Ensure that the docstrings in the following `set(... CACHE ...)`
 # commands match those in the root CMakeLists.txt file.