Turn on compiler warnings by default for AC_PROG_CC, AC_PROG_CXX & AC_PROG_FC

[Dale Visser]

This has been on my back burner for a while, but I have finally taken the time to address some concerns raised earlier.  To remind you all, these changes are about turning on a pragmatic set of compiler warning flags for C, C++ and Fortran projects by default. The changes now include an easy mechanisms for:

* disabling this new default behavior
* appending additional flags, which get checked for acceptance by the current compiler

My ability to test all combinations is limited, but I *have* tested on Linux against Gnu, LLVM and Intel compilers.

Best regards,
Dale Visser

default_compiler_warnings.diff
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