[kevin-sellers]

Object files get a target's dependencies, and its actions and rules outputs, as
order-only prerequisites. The .gch built from a prefix header gets neither, so
a prefix header that reaches a generated header races code generation: make
starts the .gch as soon as the prefix header itself exists, and a clean build
fails on the missing generated header.

This gives the .gch targets the same order-only prerequisites the object files
get.

Motivation

I was attempting to enable precompiled headers for V8's largest targets in a
Node.js build. The prefix header includes isolate-inl.h, which reaches
builtins-definitions.h, which includes the generated
builtins-generated/bytecodes-builtins-list.h. A clean build died with:

../deps/v8/src/builtins/builtins-definitions.h:8:10: fatal error:
    builtins-generated/bytecodes-builtins-list.h: No such file or directory

Any target whose GCC_PREFIX_HEADER transitively includes generated code hits
this today, independently of how the prefix header was configured.

Follow-up commits

Pushed by @gengjiawen after review:

• 0acf212: the order-only prerequisites are now written in WritePchTargets,
  so all four .gch rules (c, cc, m, mm) get them, not only those of languages
  that have objects, and the .gch rules set TOOLSET as the objects do. The
  ninja .gch builds raced code generation the same way, and now get the
  objects' predepends.
• 576b074: cmd_pch_c compiled the C prefix header with
  $(CXXFLAGS.$(TOOLSET)); it now uses $(CFLAGS.$(TOOLSET)), as the C objects
  that include it do.
• f51974c: with GCC_PRECOMPILE_PREFIX_HEADER, .s/.S sources got the
  -include of the C .gch, which nothing built for them, so a target with
  assembly and no C sources failed with pch-c/... file not found. There is no
  compiled header for assembly, so they now include the prefix header itself,
  as without precompilation.

Testing

TestGypMac.test_make_precompiled_prefix_header and
TestGypMac.test_ninja_precompiled_prefix_header generate
test/fixtures/pch_prefix.gyp, then build each .gch (c, cc, m, mm) and a .S
object on its own, each from a clean build. The prefix header of pch_action
includes a header generated by the target's action, and that of pch_dep one
generated by a dependency, so each build fails unless it waits for its
generated header. The make build also sets CXXFLAGS=-std=c++17, which a C
compile rejects.

BEGIN_COMMIT_OVERRIDE
fix: build precompiled headers after generated inputs (#360)
fix(make): compile the C prefix header with CFLAGS (#360)
fix: include the prefix header itself in assembly sources (#360)
END_COMMIT_OVERRIDE

[gengjiawen]

LGTM, thanks @kevin-sellers! I pushed follow-up commits for the ninja generator, the C PCH flags, and assembly sources; see the PR description.