Okay,  bre install pkg-config worked.

Then the installation of ExtUtils::PkgConfig also worked, so I tried to finally install the package that I actually want, namely "GD". By typing in  cpanm GD I got the message configuring GD-2.66 ... N/A

configure failed

log file:

cpanm (App::cpanminus) 1.7043 on perl 5.018002 built for darwin-thread
+-multi-2level
Work directory is /Users/my_name/.cpanm/work/1501260780.37428
You have make /usr/bin/make
You have LWP 6.05
You have /usr/bin/tar: bsdtar 2.8.3 - libarchive 2.8.3
You have /usr/bin/unzip
Searching GD () on cpanmetadb ...
--> Working on GD
Fetching http://www.cpan.org/authors/id/R/RU/RURBAN/GD-2.66.tar.gz
-> OK
Unpacking GD-2.66.tar.gz
Entering GD-2.66
Checking configure dependencies from META.json
Checking if you have ExtUtils::PkgConfig 0 ... Yes (1.16)
Checking if you have ExtUtils::Constant 0.22 ... Yes (0.23)
Configuring GD-2.66
Running Makefile.PL
Notice: Type perl Makefile.PL -h for command-line option summary.

Package gdlib was not found in the pkg-config search path.
Perhaps you should add the directory containing `gdlib.pc'
to the PKG_CONFIG_PATH environment variable
No package 'gdlib' found
 at Makefile.PL line 441.
*** can not find package gdlib
*** check that it is properly installed and available in PKG_CONFIG_PA
+TH
 at Makefile.PL line 441.
-> N/A
-> FAIL Configure failed for GD-2.66. See /Users/my_name/.cpanm/work/1
+501260780.37428/build.log for details.
[download]