In order to build for Windows under the MSYS2 shell using the MinGW-w64 GCC compiler I encountered the following issues:
-no-undefined which is required on Windowscss_parseyylex/init_yylex/end_yylexI was able to build anyway working around these issues like this:
INSTALLPREFIX=/usr/local
./configure --prefix=$INSTALLPREFIX &&
sed -i.bak -e "s/\(allow_undefined=\)yes/\1no/" libtool &&
make install-strip am_libcss_parser_pp_la_OBJECTS="parser_pp.lo parser.lo css_syntax.lo css_lex.lo" LIBS="-Wl,--as-needed -liconv" &&
echo Success