Fix compiler identification for flag selection

The name "clang" matches "cl" so we can no longer use that ancient
method of testing for MSVC.  Use modern compiler identification tests to
add compiler-specific C flags for building GCC code.

Reported-by: Sebastien Binet <binet@cern.ch>
Fix compilation with Clang

Compile GCC *.c source files with "-std=c89" since the sources use
constructs not valid under C99.

Reported-by: Sebastien Binet <binet@cern.ch>