make: Add support for automated pch generation through GCC_PREFIX_HEADER on mac.
make: Add support for automated pch generation through GCC_PREFIX_HEADER on mac.

Precompiled headers are language-dependent, so in general there can be four
precompiled headers for a single GCC_PREFIX_HEADER: c, cc, m, mm.

If a target uses GCC_PREFIX_HEADER, all the .c files depend on the
c-precompiled header, all the .cc files on the cc-precompiled header,
and so on.

The appropriate '-include path/to/header.h' is added to the cflags of
every source file. To compile the header files themselves, new flags
GYP_PCH_OBJCFLAGS (and cc, m, mm) are added, because the pch file
must not get the -include parameter, and it needs an explicit language.

With this, webcore_platform builds on mac.
Review URL: http://codereview.chromium.org/7385001

git-svn-id: e7e1075985beda50ea81ac4472467b4f6e91fc78@947 78cadc50-ecff-11dd-a971-7dbc132099af

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