Make boost::system optional

This is a header-only component since Boost 1.69 and the stub library
is no longer available since 1.89, which breaks builds requiring it.
This makes it optional to allow building with older versions of
Boost. (innoextract doesn't use it directly so it could probably be
removed entirely, but I haven't checked builds with older versions of
Boost.)

See https://www.boost.org/releases/1.89.0/ for details.

Signed-off-by: Stephen Kitt <skitt@debian.org>

CMakeLists.txt

@@ -177,8 +177,9 @@ find_package(Boost REQUIRED COMPONENTS
 	iostreams
 	filesystem
 	date_time
-	system
 	program_options
+	OPTIONAL_COMPONENTS
+	system
 )
 list(APPEND LIBRARIES ${Boost_LIBRARIES})
 link_directories(${Boost_LIBRARY_DIRS})

README.md

@@ -21,7 +21,7 @@ Author: [Daniel Scharrer](https://constexpr.org/)
 * **liblzma** from [xz-utils](https://tukaani.org/xz/) *(optional)*
 * **iconv** (*optional*, either as part of the system libc, as is the case with [glibc](https://www.gnu.org/software/libc/) and [uClibc](https://uclibc.org/), or as a separate [libiconv](https://www.gnu.org/software/libiconv/))
 
-For Boost you will need the headers as well as the `iostreams`, `filesystem`, `date_time`, `system` and `program_options` libraries. Older Boost version may work but are not actively supported. The boost `iostreams` library needs to be build with zlib and bzip2 support.
+For Boost you will need the headers as well as the `iostreams`, `filesystem`, `date_time` and `program_options` libraries. Older Boost versions may work but are not actively supported. The boost `iostreams` library needs to be built with zlib and bzip2 support.
 
 While innoextract can be built without liblzma by manually setting `-DUSE_LZMA=OFF`, it is highly recommended and you won't be able to extract most installers created by newer Inno Setup versions without it.