CMake build broken, tries to install man files in /man not in CMAKE_INSTALL_PREFIX

Hi, I'm building brotli using the following configuration

cmake -DCMAKE_INSTALL_PREFIX:PATH=$3 -DINSTALL_PKGCONFIG_DIR=$3/lib/pkgconfig -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=OFF ../${SOFTWARE}
checkStatus $? "configuration of ${SOFTWARE} failed"

$3 is /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool

${SOFTWARE} = brotli

And it tried to install the man files in root not the Install destination

Consolidate compiler generated dependencies of target brotli
[100%] Built target brotli
Install the project...
-- Install configuration: "Release"
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/bin/brotli
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/lib/libbrotlienc.a
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/lib/libbrotlidec.a
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/lib/libbrotlicommon.a
-- Up-to-date: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/include/brotli
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/include/brotli/shared_dictionary.h
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/include/brotli/port.h
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/include/brotli/types.h
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/include/brotli/encode.h
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/include/brotli/decode.h
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/lib/pkgconfig/libbrotlicommon.pc
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/lib/pkgconfig/libbrotlidec.pc
-- Installing: /Users/xxxx/SSD/Source/BuildTest/build-ffmpeg/tool/lib/pkgconfig/libbrotlienc.pc
CMake Error at cmake_install.cmake:99 (file):
file cannot create directory: /man/man1. Maybe need administrative
privileges.

This used to work, last confirmed successful build 16th August 2023