[ruby] fix re2 compilation when older system version installed (#32580)

re2 previously failed to compile if:

1. An old `re2` version is installed with a non-standard system prefix,
such as `/opt/local`.
2. The environment variable is set: `CPPFLAGS=-I/opt/local/include`.

Running `make` would result in function prototype mismatches because the
Makefile would previously attempt to use the headers from
`/opt/local/include/re2` before the `third_party/re2/re2` directory.

https://github.com/grpc/grpc/pull/27660 caused `CPPFLAGS` to inherit
from the environment, but this can cause the Makefile to use external
include files for re2 and other libraries if `-I` flags are defined.

This commit reverts to the original behavior of only using
`RbConfig::CONFIG` values to avoid using the wrong headers.

src/ruby/ext/grpc/extconf.rb

@@ -34,24 +34,23 @@ def env_unset?(name)
   ENV[name].nil? || ENV[name].size == 0
 end
 
-def rbconfig_set?(name)
-  RbConfig::CONFIG[name] && RbConfig::CONFIG[name].size > 0
+def inherit_env_or_rbconfig(name)
+  ENV[name] = inherit_rbconfig(name) if env_unset?(name)
 end
 
 def inherit_rbconfig(name)
-  ENV[name] = RbConfig::CONFIG[name] if env_unset?(name) && rbconfig_set?(name)
+  ENV[name] = RbConfig::CONFIG[name] || ''
 end
 
 def env_append(name, string)
-  ENV[name] ||= ''
   ENV[name] += ' ' + string
 end
 
-inherit_rbconfig 'AR'
-inherit_rbconfig 'CC'
-inherit_rbconfig 'CXX'
-inherit_rbconfig 'RANLIB'
-inherit_rbconfig 'STRIP'
+inherit_env_or_rbconfig 'AR'
+inherit_env_or_rbconfig 'CC'
+inherit_env_or_rbconfig 'CXX'
+inherit_env_or_rbconfig 'RANLIB'
+inherit_env_or_rbconfig 'STRIP'
 inherit_rbconfig 'CPPFLAGS'
 inherit_rbconfig 'LDFLAGS'