Installing R package XML fails despite having all dependencies

Question

Running on Linux LEAP, R 4.1.2 (latest official release for this OS) Trying to install (in Rstudio ) R package XML. It fails with this (relevant lines):

Checking directory of LIBXML_INCDIR
Located parser file /usr/include/libxml2/parser.h
Checking for 1.8:  /usr/include/libxml2
Using libxml2.*
checking for gzopen in -lz... no
checking for xmlParseFile in -lxml2... no
checking for xmlParseFile in -lxml... no
configure: error: "libxml not found"
ERROR: configuration failed for package ‘XML’

But I HAVE zlib & libxml2 installed & up to date. Have specified correct paths for these libraries, and some other flags, in ~/.Renviron :

LIBXML_INCDIR=/usr/include/libxml2
LIBXML_LIBDIR=/usr/lib64
PKG_CONFIG_PATH=/usr/lib64/pkgconfig
PKG_LIBS=-L/lib64 -lz
PKG_LIBDIR=/lib64
LDFLAGS=-L/usr/include/libxml2
CPPFLAGS=-I/usr/include/libxml2

And verified with Sys.getenv that they are set correctly in the R session, before running the install. I think the output lines:

Located parser file /usr/include/libxml2/parser.h
Checking for 1.8:  /usr/include/libxml2
Using libxml2.*

imply that it is looking in the right place and HAS found libxml2. Yet it then cant find xmlParseFile and exits with "libxml not found".

Things I tried (with a little bit of help from chatgpt...)

1. Setting config vars in the R session and in the install.packages command itself ( configure.args or configure.vars)
2. Installing from source
3. Installing from source in bash with R CMD INSTALL (after exporting config vars in bash shell)
4. installing from github using remotes package Everything gives the same error message. Not managing to get more info on the error - bash install doesnt produce a config.log , and setting Sys.setenv(R_INSTALL_STAGED = "yes") gives no additional info . Am totally stumped! any ideas would be so much appreciated...