precompiled header suggestion

[Tom Tromey]

Recently gcc added precompiled header support.  This is mostly useful
for C++, but C might benefit in some cases too.

To use it, you make a special `.gch' file by compiling a bunch of .h
files.  Then you tell gcc to use it when compiling.

Automake could usefully automate this.  First, when building the .gch
file we could do automatic dependency tracking (the process of
building this file should support the normal -M flags).

Also, if a .gch file is listed in _SOURCES, it would be cool to build
this file before trying to build any of the objects associated with
the _SOURCES variable.  (This could be generalized to all .h files,
perhaps, to let us reduce the scope of BUILT_SOURCES.)  This could be
implemented by adding a new dependency for each .o file.

There would also have to be a way to disable .gch support for non-gcc
compilers.

Tom