Am 16.09.2017 um 19:14 schrieb Artur T.:
 Hi Helio, thank for your reply and the suggestion. I guess this would
 have been the most simple way to solve this issue.
 
 However, meanwhile I've found another workaround: I found out that I can 
 tell gretl the correct paths in the file /lib/.deps/gretl_foreign.d. 
 Unfortunately, this has to be done by hand but at least the installation 
 works well. 
I don't know, this sounds very error-prone (and apparently in the end 
you get an error).
The Gretl guide in chapter 36.6 says "you should ensure [...] (b) that 
the pkg-config program is able to detect your R installation."
So perhaps you should set the environment variable PKG_CONFIG_PATH 
(which comes with the explanation "directories to add to pkg-config's 
search path")  to point to your special R install directory.
good luck,
sven