Doc building not exposed in cmake build path

[LecrisUT]

This is done in meson with

mctc-lib/meson.build

Lines 124 to 133 in 93dbc89

	asciidoc = find_program('asciidoctor', required: false)
	if asciidoc.found()
	install_man(
	configure_file(
	command: [asciidoc, '-b', 'manpage', '@INPUT@', '-o', '@OUTPUT@'],
	input: files('man/mctc-convert.1.adoc'),
	output: '@BASENAME@',
	)
	)
	endif

and this can be included in cmake quite easily

find_program(ASCIIDOC_EXEC asciidoctor)
add_custom_target(man
    COMMAND ${ASCIIDOC_EXEC} -b manpage man/mctc-convert.1.adoc -o mctc-convert
)

install(
  FILES
  "${CMAKE_CURRENT_BINARY_DIR}/mctc-convert.1"
  DESTINATION "${CMAKE_INSTALL_MANDIR}/man1"
)