Allow GoogleTest to find clone() with PGI compiler

[GoogleCodeExporter]

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If you really need to create a new issue, please provide the information
asked for below.

What steps will reproduce the problem?
1. Compile gmock-1.7.0 with PGI C++ compiler on Linux
2. Observe failure to find clone()
3.

What is the expected output? What do you see instead?

clone() should be found, or the fallback triggered.

What version of Google Test are you using? On what operating system?

1.7.0, PGI C++ compiler, Linux

Please provide any additional information below, such as a code snippet.

To fix, either #define _GNU_SOURCE if __PGI is defined, so that the compiler 
will find clone() in sched.h, or trigger GoogleTest's the fork() fallback for 
clone() in this case.

Original issue reported on code.google.com by Mark.J.A...@gmail.com on 16 Aug 2014 at 11:04

[GoogleCodeExporter]

Or perhaps use the appropriate feature-test macro before assuming that a 
compiler on Linux always provides _GNU_SOURCE.

Original comment by Mark.J.A...@gmail.com on 20 Aug 2014 at 4:37