./configure supports using the system or third-party-supplied pkg-config utility to find or override the default location of header and library files when building a few C extensions in the standard library, namely from OpenSSL for the _ssl module, libffi for ctypes, and, new in 3.10, Tcl/Tk for _tkinter (bpo-42603). However, currently for 3.10.0, pkg-config usage is broken for libffi and Tcl/Tk (but not OpenSSL). When running ./configure, there is an unexpected warning that is easily overlooked:

[...]
checking for --with-libs... no
./configure: line 10545: PKG_PROG_PKG_CONFIG: command not found
checking for --with-system-expat... no
[...]

PKG_PROG_PKG_CONFIG is a macro provided by GNU Autotools that is supposed to be in aclocal.m4. Unfortunately, it appears to have been inadvertently deleted in 2fc857a5721a5b42bcb696c9cae1bbcc82a91b17 (PR 25860) probably due to an autoconf version mismatch. The net effect is that the configure variable PKG_CONFIG, the location of the pkg-config utility, is undefined so tests in configure for the location of libffi and of Tcl and Tk do not take into account any pkg-config info and use any default locations (i.e. /usr/include). For most builds, that likely still produces the desired results. But it won't if a builder is trying to override these locations or is building on a platform with different default or is using a third-party package manager and pkg-config to supply libraries.  Note, the _ssl module builds are not affected by this problem as the AX_CHECK_OPENSSL macro in aclocal.m4 does not depend on PKG_PROG_PKG_CONFIG to find pkg-config.

It appears that the problem can be worked around by explicitly setting the PKG_CONFIG build variable when invoking configure, something like:

  ./configure [...] PKG_CONFIG=$(which pkg-config)

But the PR 25860 changes to aclocal.a4 should be carefully reviewed and the pkg-config related deletes restored; there might be other problems, too. This is not the first time we've been caught up by unexpected autoconf changes and, as is clear here, it is too easy for them to go unnoticied. Perhaps we should try to figure out how to reduce those risks.