WARNINGS_AS_ERRORS is ON/OFF, not TRUE/FALSE

[AE1020]

Amends outdated README-CMake.md. Relevant CMake code here:

z3/cmake/compiler_warnings.cmake

Lines 130 to 158 in 49a0719

	if (WARNINGS_AS_ERRORS STREQUAL "ON")
	message(STATUS "Treating compiler warnings as errors")
	if ((CMAKE_CXX_COMPILER_ID MATCHES "Clang") OR (CMAKE_CXX_COMPILER_ID MATCHES "GNU"))
	list(APPEND Z3_COMPONENT_CXX_FLAGS "-Werror")
	elseif (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
	list(APPEND Z3_COMPONENT_CXX_FLAGS "/WX")
	else()
	message(AUTHOR_WARNING "Unknown compiler")
	endif()
	elseif (WARNINGS_AS_ERRORS STREQUAL "SERIOUS_ONLY")
	message(STATUS "Treating only serious compiler warnings as errors")
	# Loop through the flags
	foreach (flag ${WARNING_AS_ERROR_FLAGS_TO_CHECK})
	# Add globally because some flags need to be passed at link time.
	z3_add_cxx_flag("${flag}" GLOBAL)
	endforeach()
	elseif (WARNINGS_AS_ERRORS STREQUAL "OFF")
	message(STATUS "Not treating compiler warnings as errors")
	if (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
	# Warnings as errors is off by default for MSVC so setting this
	# is not necessary but this duplicates the behaviour of the old
	# build system.
	list(APPEND Z3_COMPONENT_CXX_FLAGS "/WX-")
	endif()
	else()
	message(FATAL_ERROR
	"WARNINGS_AS_ERRORS set to unsupported value \"${WARNINGS_AS_ERRORS}\""
	)
	endif()

However, ON doesn't work with GCC, due to unused-function warnings, mentioned as lingering in #4463 (comment)

[[maybe_unused]] annotations do not appear to help... seems to have something to do with explicit template instantiation