Revert "compat/strdup.h: move common compat check for strdup() to own file"

Revert "compat/strdup.h: move common compat check for strdup() to own file"

This reverts commit aaba8c1080533133329086e47f4f45981dfdd1ee.

This commit breaks builddir != srcdir build, but, more importantly, it
also adds a dependency on a header, "config.h", which is not installed
and it's supposed to be private — since it's generated at configuration
time and it's not meant to be used by projects compiling against a
library.