ports that use use_xmkmf yes fail with make Makefiles failure

[ryandesign (Ryan Carsten Schmidt)]

Ports that use use_xmkmf yes fail to build. For example:

$ sudo port -n build tgif
--->  Cleaning tgif
--->  Computing dependencies for tgif
--->  Fetching distfiles for tgif
--->  Verifying checksums for tgif
--->  Extracting tgif
--->  Applying patches to tgif
--->  Configuring tgif
Error: Failed to configure tgif: make Makefiles failure: can't read "cd /opt/local/var/macports/build/_Users_rschmidt_macports_macports-ports-ryandesign-fork_graphics_tgif/tgif/work/tgif-QPL-4.2.5 && make Makefiles.dir": no such variable
Error: See /opt/local/var/macports/logs/_Users_rschmidt_macports_macports-ports-ryandesign-fork_graphics_tgif/tgif/main.log for details.
Error: Follow https://guide.macports.org/#project.tickets if you believe there is a bug.
Error: Processing of port tgif failed

This has the same root cause as #63931: as of [da3524e8a3006a9b9a2370b66fa91bb554dd768b/macports-base] (which introduced build progress bars) which was included for the first time in MacPorts 2.7.0, command_exec requires flags to be specified before the command but not all uses of command_exec were adjusted to match this new requirement; the invocation of command_exec for running make Makefiles when using xmkmf was not.

In addition, command_exec no longer honors the -varprefix flag which use_xmkmf yes requires.

[ryandesign (Ryan Carsten Schmidt)]

In c8ff808191a16032509ff5bf5b2df35faa9ed4c7/macports-base (master):

Fix ports that use "use_xmkmf yes"

This was broken by da3524e which changed command_exec to require flags
to be specified before the command without updating all uses of
command_exec to conform to the new requirement.

Closes: #64011