Bad gcc/gtype-desc.h generated when using sparse checkout

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

At some point in the past few weeks it became impossible to build
trunk from a sparse checkout that omits certain directories.

Because I nearly always configure with --enable-languages=c,c++ I use
git's core.sparseCheckout=true config or "svn update --set-depth" to
avoid checking out the front-ends, libs and tests for java, fortran
and ada.  This has always worked fine until recently, when it resulted
in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53832

/home/jwakely/src/gcc/gcc/cp/lex.c:559:41: error: macro
"ggc_alloc_cleared_lang_decl" passed 1 arguments, but takes just 0
/home/jwakely/src/gcc/gcc/cp/lex.c: In function 'retrofit_lang_decl':
/home/jwakely/src/gcc/gcc/cp/lex.c:559: error: 'ggc_alloc_cleared_lang_decl'
undeclared (first use in this function)

I know what I'm trying to do isn't supported, but if the contents of
gcc/ada and gcc/fortran are required to build, shouldn't it be a hard
error if they're missing, rather than generating an unusable
gtype-desc.h file?