GN: nacl_defines: Drop _BSD_SOURCE, _DEFAULT_SOURCE; conditionalize on OS
GN: nacl_defines: Drop _BSD_SOURCE, _DEFAULT_SOURCE; conditionalize on OS

Drop _BSD_SOURCE and _DEFAULT_SOURCE defines, which are redundant with
_GNU_SOURCE on Linux.  This brings the set affecting Linux in line with
the standalone NaCl build.

Conditionalize these defines to affect only Linux, Android, and NaCl
toolchains.  That is, none of these are used on Mac (or Windows).  This
makes the GN build consistent with the GYP and Scons builds.  On Mac,
defining _POSIX_C_SOURCE (without also defining _DARWIN_C_SOURCE) makes
the system headers disable some symbols that NaCl sources need.

BUG=545236
R=dpranke@chromium.org, mseaborn@chromium.org

Review URL: https://codereview.chromium.org/1408273003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355327}

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