Detect and report incompatible gdb_compile options

Detect and report incompatible gdb_compile options

In commits 221db974e653659edb280787af1b3efdd1615083 and
68d654afdfcff840ebb3ae432ed72dca0521d670, these patches:

    2020-06-24  Pedro Alves  <palves@redhat.com>

	* lib/gdb.exp (gdb_compile): Pass "-x c++" explicitly when
	compiling C++ programs.

    2020-09-25  Gary Benson <gbenson@redhat.com>

	* lib/gdb.exp (gdb_compile): Pass "-x c++" earlier, and only
	for .c files.

attempted to fix problems with testcases that compile .c files
using the C++ compiler.  These patches cause gdb_compile to add
"-x c++" to the compiler options when using Clang.  This fix does
not work for gdb.base/print-file-var.exp, however, which attempts
to compile a .c input file to an executable linked with shared
libraries: the resulting command caused the compiler to attempt
to parse the .so files as C++.  This commit causes gdb_compile
to reject this combination of options.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (gdb_compile): Inhibit passing "-x c++"
	for .c files compiled as C++ with Clang if any shared
	libraries are specified.