The /usr/lib64/pkgconfig/libbrotlicommon.pc, /usr/lib64/pkgconfig/libbrotlienc.pc and /usr/lib64/pkgconfig/libbrotlidec.pc pkg-config files contains this link-time option in the Libs field, in addition to the more normal -L link-time option:
gcc does not accept the -R option and this causes amongst other things libsoup to fail to compile. Omitting the -R option enables compilation to proceed.