CMakeLists.txt: use `CMAKE_INSTALL_FULL_MANDIR` for mans install

Without the change install just fails for me as `SHARE_INSTALL_PREFIX`
is unset for me.

Following https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
I'm using absolute path expansion to install mans.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f13b62c..ffb640c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -359,9 +359,9 @@
     DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
 endif()  # BROTLI_BUNDLED_MODE
 
-INSTALL(FILES "docs/brotli.1" DESTINATION "${SHARE_INSTALL_PREFIX}/man/man1")
+INSTALL(FILES "docs/brotli.1" DESTINATION "${CMAKE_INSTALL_FULL_MANDIR}/man1")
 INSTALL(FILES docs/constants.h.3 docs/decode.h.3 docs/encode.h.3 docs/types.h.3
-  DESTINATION "${SHARE_INSTALL_PREFIX}/man/man3")
+  DESTINATION "${CMAKE_INSTALL_FULL_MANDIR}/man3")
 
 if (ENABLE_COVERAGE STREQUAL "yes")
   SETUP_TARGET_FOR_COVERAGE(coverage test coverage)