cmake: Add docstrings for options when cross-compiling

[hebasto]

See discussion in #118 (comment).

Can be tested in the following ways:

• applying the -LH command line option
• using cmake-gui
• direct observing of the CMakeCache.txt file content

[hebasto]

Friendly ping @vasild @m3dwards @pablomartin4btc @TheCharlatan :)

[pablomartin4btc]

tACK 840e494

Tested using -LH command line option (list options with human-readable help) on the cmake configured with --toolchain after cross-building depends.

...

-- Cache values
// Build bench_bitcoin executable.
BUILD_BENCH:BOOL=ON

// Build bitcoin-cli executable.
BUILD_CLI:BOOL=ON

// Build bitcoind executable.
BUILD_DAEMON:BOOL=ON

// Build fuzz binary.
BUILD_FUZZ_BINARY:BOOL=ON

// Build test_bitcoin-qt executable.
BUILD_GUI_TESTS:BOOL=ON

// Build the testing tree.
BUILD_TESTING:BOOL=ON

// Build test_bitcoin executable.
BUILD_TESTS:BOOL=ON

// Build bitcoin-tx executable.
BUILD_TX:BOOL=ON

// Build bitcoin-util executable.
BUILD_UTIL:BOOL=ON

// Build bitcoin-wallet tool.
BUILD_WALLET_TOOL:BOOL=ON

// Use ccache for compiling. "AUTO" means "ON" if ccache is found.
CCACHE:STRING=AUTO

// Choose the type of build, options are: None Debug Release RelWithDebInfo MinSizeRel ...
CMAKE_BUILD_TYPE:STRING=RelWithDebInfo

// Convert GNU import libraries to MS format (requires Visual Studio)
CMAKE_GNUtoMS:BOOL=OFF

// Install path prefix, prepended onto install directories.
CMAKE_INSTALL_PREFIX:PATH=/usr/local

// The CMake toolchain file
CMAKE_TOOLCHAIN_FILE:FILEPATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/share/toolchain.cmake

// Enable wallet.
ENABLE_WALLET:BOOL=ON

// Build for fuzzing. Enabling this will disable all other targets and override BUILD_FUZZ_BINARY.
FUZZ:BOOL=OFF

// Attempt to harden the resulting executables.
HARDENING:BOOL=ON

// Install man pages.
INSTALL_MAN:BOOL=ON

// The directory containing a CMake configuration file for QT.
QT_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5

// The directory containing a CMake configuration file for Qt5AccessibilitySupport.
Qt5AccessibilitySupport_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5AccessibilitySupport

// The directory containing a CMake configuration file for Qt5Core.
Qt5Core_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5Core

// The directory containing a CMake configuration file for Qt5EventDispatcherSupport.
Qt5EventDispatcherSupport_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5EventDispatcherSupport

// The directory containing a CMake configuration file for Qt5FontDatabaseSupport.
Qt5FontDatabaseSupport_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5FontDatabaseSupport

// The directory containing a CMake configuration file for Qt5Gui.
Qt5Gui_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5Gui

// The directory containing a CMake configuration file for Qt5LinguistTools.
Qt5LinguistTools_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5LinguistTools

// The directory containing a CMake configuration file for Qt5Network.
Qt5Network_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5Network

// The directory containing a CMake configuration file for Qt5Test.
Qt5Test_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5Test

// The directory containing a CMake configuration file for Qt5ThemeSupport.
Qt5ThemeSupport_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5ThemeSupport

// The directory containing a CMake configuration file for Qt5Widgets.
Qt5Widgets_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5Widgets

// The directory containing a CMake configuration file for Qt5WindowsUIAutomationSupport.
Qt5WindowsUIAutomationSupport_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5WindowsUIAutomationSupport

// The directory containing a CMake configuration file for Qt5.
Qt5_DIR:PATH=/home/pablo/workspace/bitcoin-hebasto/depends/x86_64-w64-mingw32/lib/cmake/Qt5

// Attempt to reduce exported symbols in the resulting executables.
REDUCE_EXPORTS:BOOL=OFF

// Enable features that depend on the C++ thread_local keyword (currently just thread names in debug logs).
THREADLOCAL:BOOL=ON

// Warn when using a Berkeley DB (BDB) version other than 4.8.
WARN_INCOMPATIBLE_BDB:BOOL=ON

// Treat compiler warnings as errors.
WERROR:BOOL=OFF

// Enable Berkeley DB (BDB) wallet support. "AUTO" means "ON" if libdb_cxx is found.
WITH_BDB:STRING=AUTO

// Enable external signer support. "AUTO" means "ON" if Boost.Process is found.
WITH_EXTERNAL_SIGNER:STRING=AUTO

// Build GUI ([AUTO], Qt5, OFF)
WITH_GUI:STRING=AUTO

// Enable UPnP. "AUTO" means "ON" if libminiupnpc is found.
WITH_MINIUPNPC:STRING=AUTO

// Enable NAT-PMP. "AUTO" means "ON" if libnatpmp is found.
WITH_NATPMP:STRING=AUTO

// Enable QR code support. "AUTO" means "ON" if libqrencode is found.
WITH_QRENCODE:STRING=AUTO

// Enable SQLite wallet support. "AUTO" means "ON" if libsqlite3 is found.
WITH_SQLITE:STRING=AUTO

// Enable tracepoints for Userspace, Statically Defined Tracing. "AUTO" means "ON" if sys/sdt.h is found.
WITH_USDT:STRING=AUTO

// Enable ZMQ notifications. "AUTO" means "ON" if libzmq is found.
WITH_ZMQ:STRING=AUTO