improve cmake toolchain variable handling and pkg-config detection

src/cmake/conf/mxe-conf.cmake.in

@@ -22,6 +22,16 @@ if(NOT ${CMAKE_COMMAND} STREQUAL @PREFIX@/@BUILD@/bin/cmake AND NOT DEFINED ENV{
     set(ENV{_MXE_CMAKE_TOOLCHAIN_INCLUDED} TRUE)
 endif()
 
+# Use CACHE variables to allow user setting with `-D`
+# Use CACHE FORCE in rare cases of misconfigured CMakeLists.txt
+#   - e.g include(FindPkgConfig)
+#     https://github.com/mxe/mxe/issues/1023
+#   - projects may still set these in which case FORCE doesn't have
+#     any advantage, just causes inconvenience
+#     https://github.com/mxe/mxe/pull/1621#discussion_r106937505
+# Use normal variables expected to be set by toolchain/system
+#   - projects should test for these values and not try to override
+
 ## General configuration
 set(CMAKE_SYSTEM_NAME Windows)
 set(CMAKE_SYSTEM_PROCESSOR @PROCESSOR@ CACHE STRING "System Processor")
@@ -32,19 +42,30 @@ set(CMAKE_CROSS_COMPILING ON)
 
 
 ## Library config
-set(BUILD_SHARED_LIBS @CMAKE_SHARED_BOOL@ CACHE BOOL "BUILD_SHARED_LIBS" FORCE)
-set(BUILD_STATIC_LIBS @CMAKE_STATIC_BOOL@ CACHE BOOL "BUILD_STATIC_LIBS" FORCE)
-set(BUILD_SHARED @CMAKE_SHARED_BOOL@ CACHE BOOL "BUILD_SHARED" FORCE)
-set(BUILD_STATIC @CMAKE_STATIC_BOOL@ CACHE BOOL "BUILD_STATIC" FORCE)
+set(BUILD_SHARED_LIBS @CMAKE_SHARED_BOOL@ CACHE BOOL "BUILD_SHARED_LIBS")
+set(BUILD_STATIC_LIBS @CMAKE_STATIC_BOOL@ CACHE BOOL "BUILD_STATIC_LIBS")
+set(BUILD_SHARED @CMAKE_SHARED_BOOL@ CACHE BOOL "BUILD_SHARED")
+set(BUILD_STATIC @CMAKE_STATIC_BOOL@ CACHE BOOL "BUILD_STATIC")
 set(LIBTYPE @LIBTYPE@)
 
 
 ## Paths etc.
-set(CMAKE_FIND_ROOT_PATH @PREFIX@/@TARGET@)
+# These MODEs shouldn't be changed by users, we only want headers/libs
+# from cross-build and "never" want binaries. We do, however, want
+# `*-config` scripts but there's no way to instruct cmake to do that.
+#
+# The best solution may be to whitelist utilities
+#   https://github.com/mxe/mxe/issues/1667
+# and symlink them to an additional root path, changing PROGRAM to ONLY
+
 set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
 set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
-set(CMAKE_PREFIX_PATH @PREFIX@/@TARGET@)
+set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
+
+# Allow user to specify list of locations to search
+set(CMAKE_FIND_ROOT_PATH @PREFIX@/@TARGET@ CACHE PATH "List of root paths to search on the filesystem")
+set(CMAKE_PREFIX_PATH @PREFIX@/@TARGET@ CACHE PATH "List of directories specifying installation prefixes to be searched")
 set(CMAKE_INSTALL_PREFIX @PREFIX@/@TARGET@ CACHE PATH "Installation Prefix")
 # For custom mxe FindPackage scripts
 set(CMAKE_MODULE_PATH "@PREFIX@/share/cmake/modules" ${CMAKE_MODULE_PATH})

src/pkgconf.mk

@@ -28,10 +28,22 @@ define $(PKG)_BUILD
     chmod 0755 '$(PREFIX)/bin/$(TARGET)-pkg-config'
 
     # create cmake file
+    # either of these before `project` command will find native
+    # `pkg-config` regardless of CACHE FORCE setting in toolchain
+    #   - find_package(PkgConfig)
+    #   - include(FindPkgConfig)
+    #
+    # it seems the `project` command loads CMAKE_TOOLCHAIN_FILE
+    # but that isn't documented anywhere
     mkdir -p '$(CMAKE_TOOLCHAIN_DIR)'
-    echo 'set(PKG_CONFIG_EXECUTABLE $(PREFIX)/bin/$(TARGET)-pkg-config CACHE PATH "pkg-config executable")' \
-    > '$(CMAKE_TOOLCHAIN_DIR)/pkgconf.cmake'
-
+    (echo 'if(PKG_CONFIG_FOUND)'; \
+     echo '  message(FATAL_ERROR "'; \
+     echo '  ** find_package(PkgConfig) or (deprecated) include(FindPkgConfig)'; \
+     echo '  ** must be invoked after project() command when using CMAKE_TOOLCHAIN_FILE'; \
+     echo '  ")'; \
+     echo 'endif()'; \
+     echo 'set(PKG_CONFIG_EXECUTABLE $(PREFIX)/bin/$(TARGET)-pkg-config CACHE PATH "pkg-config executable")'; \
+    )> '$(CMAKE_TOOLCHAIN_DIR)/pkgconf.cmake'
 endef
 
 define $(PKG)_BUILD_$(BUILD)