Silence warnings about -Wno-deprecated-non-prototype (#989)

Builds were producing a great number of warnings like this:

```
cc1plus: warning: command-line option '-Wno-deprecated-non-prototype' is valid for C/ObjC but not for C++
```

These warnings came from a build option in the `.bazelrc` file written
by `configure.sh`, specifically the application of
`-Wno-deprecated-non-prototype` to C++ files in external dependencies
even though the flag is only valid for C. This PR changes the Bazel
build `per_file_copt` and `host_per_file_copt` regexes for
`-Wno-deprecated-non-prototype` from `external/.*` to `external/.*[.]c$`
to target only C source files.

Author: mhucka

configure.sh

@@ -233,13 +233,15 @@ if ! is_windows; then
   write_bazelrc "build --linkopt=-Wl,-rpath,${LIBDIR}"
 fi
 
+write_bazelrc ""
+
 # The following supressions are for warnings coming from external dependencies.
 # They're most likely inconsequential or false positives. Since we can't fix
-# them, we suppress the warnings to reduce noise during builds.
+# them, we suppress the warnings to reduce noise. Note: single quotes are needed
+# for the first two so that the $ anchors are preserved in the .bazelrc file.
 
-write_bazelrc ""
-write_bazelrc "build --per_file_copt=external/.*@-Wno-deprecated-non-prototype"
-write_bazelrc "build --host_per_file_copt=external/.*@-Wno-deprecated-non-prototype"
+write_bazelrc 'build --per_file_copt=external/.*[.]c$@-Wno-deprecated-non-prototype'
+write_bazelrc 'build --host_per_file_copt=external/.*[.]c$@-Wno-deprecated-non-prototype'
 write_bazelrc "build --per_file_copt=external/com_google_protobuf/.*@-Wno-unused-function"
 write_bazelrc "build --host_per_file_copt=external/com_google_protobuf/.*@-Wno-unused-function"
 write_bazelrc "build --per_file_copt=external/eigen/.*@-Wno-maybe-uninitialized"