[dev-context] mptopdf says 'Unable to make mpx file' (using TEX.mp)

[Sanjoy Mahajan]

Here is 'minimal.mp':

input TEX
beginfig(1)
  draw thelabel.top(TEX("90"), origin);
endfig;
end

It runs fine through mpost, but running 'mptopdf minimal.mp' instead
produces:

  MPtoPDF 1.3.3 : running 'texexec --mptex    minimal.mp'
  TeXExec | processing graphic 'minimal.mp'
  This is MetaPost, Version 0.993 (Web2C 7.5.6)
  (/usr/share/texmf/web2c/natural.tcx)
  (minimal.mp (/usr/share/texmf-texlive/metapost/base/TEX.mp) (mptextmp.mp
  >> mptextmp.mp
  >> mptextmp.mpx
  ! Unable to make mpx file.
  l.1 btex
	   90 etex
  Transcript written on minimal.log.
  TeXExec | runtime: 0.040707

  MPtoPDF 1.3.3 : no filename matches minimal

(After running mpost, the mptopdf command works fine, so delete all
 files except minimal.mp, then run 'mptopdf minimal.mp' to see the
 problem.)

I'm using the Debian unstable texlive/context packages on i386 (using
mkii).  Do others see this problem?  I don't think it's related to the
mpost version or to the Debian packaging.  I just tried a moderately
recent 'minimals' and mpost 1.110, and the problem is the same.

-Sanjoy