I was trying to compile mod_openpgp under Fedora 7 x86_64 with command:

apxs -c -Wc,-g -i -l gpgme -l gpg-error -I /usr/include/ mod_openpgp.c

Output was:

cannot open /httpd/build/config_vars.mk: No such file or directory at /usr/sbin/apxs line 201.

The problem caused because pkg-config --variable=libdir apr-1 was giving just nothing. It happens because I had installed apr-devel.i386 and httpd-devel.x86_64.
Problem solved by removing i386 apr-devel and installing x86_64 version instead