Based on remove-gnu99.patch from
https://src.fedoraproject.org/rpms/grpc/pull-request/3; corresponds to ignored
upstream PR https://github.com/grpc/grpc/pull/23671. For consistency, we also
patch a related shell script that we do not use.

Using -std=gnu99 for C++ code makes GCC warn and clang error; besides, it makes
no sense.

diff -Naur grpc-1.26.0-original/setup.py grpc-1.26.0/setup.py
--- grpc-1.26.0-original/setup.py	2019-12-17 20:29:27.000000000 -0500
+++ grpc-1.26.0/setup.py	2021-02-01 11:49:55.741613488 -0500
@@ -179,7 +179,7 @@
       # available dynamically
       EXTRA_ENV_COMPILE_ARGS += ' /MT'
   elif "linux" in sys.platform:
-    EXTRA_ENV_COMPILE_ARGS += ' -std=gnu99 -fvisibility=hidden -fno-wrapv -fno-exceptions'
+    EXTRA_ENV_COMPILE_ARGS += ' -fvisibility=hidden -fno-wrapv -fno-exceptions'
   elif "darwin" in sys.platform:
     EXTRA_ENV_COMPILE_ARGS += ' -stdlib=libc++ -fvisibility=hidden -fno-wrapv -fno-exceptions'
 
diff -Naur grpc-1.26.0-original/tools/run_tests/helper_scripts/build_python.sh grpc-1.26.0/tools/run_tests/helper_scripts/build_python.sh
--- grpc-1.26.0-original/tools/run_tests/helper_scripts/build_python.sh	2019-12-17 20:29:27.000000000 -0500
+++ grpc-1.26.0/tools/run_tests/helper_scripts/build_python.sh	2021-02-01 11:49:37.059492917 -0500
@@ -116,7 +116,7 @@
 fi
 
 ROOT=$(pwd)
-export CFLAGS="-I$ROOT/include -std=gnu99 -fno-wrapv $CFLAGS"
+export CFLAGS="-I$ROOT/include -fno-wrapv $CFLAGS"
 export GRPC_PYTHON_BUILD_WITH_CYTHON=1
 export LANG=en_US.UTF-8