Add platform detection logic for SecureRandom (#676)

* Add CMake platform detection logic for SecureRandom

Now only builds secure_random_arc4random.cc if available.

Remove firebase/firestore/base/port.h. Nothing else was in that
directory.

* Add a SecureRandom implementation that uses OpenSSL

This is usable on Linux, Windows, and Android

* Properly check return from RAND_bytes

1 files changed, 3 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
index 79daac4..7645c89 100644
--- a/.gitignore
+++ b/.gitignore
@@ -58,6 +58,9 @@ Pods/
 Podfile.lock
 *.xcworkspace
 
+# Firestore's build configuration, as generated by CocoaPods
+Firestore/core/src/firebase/firestore/util/config.h
+
 # CMake
 .downloads
 Debug