Check if cmake is installed on RNTester (#39208)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39208

To properly build RNTester for iOS, we need cmake installed.
This small change check whether Cmake is present in the system. If not, it tries to install it using `brew` if that's available.
Otherwise, it exits with an error message asking to install cmake.

## Changelog:
[Internal] -  Check whether Cmake is present for RNTester

Reviewed By: dmytrorykun

Differential Revision: D48821932

fbshipit-source-id: 94f1000a22a9f9d06efdec70b37698b99af358e5

packages/rn-tester/Podfile

@@ -3,6 +3,22 @@ require_relative '../react-native/scripts/react_native_pods'
 source 'https://cdn.cocoapods.org/'
 platform :ios, min_ios_version_supported
 
+cmake_path = `command -v cmake`
+
+if cmake_path == ""
+  brew_path = `command -v brew`
+  if brew_path != ""
+    Pod::UI.puts "Installing CMake using brew. This is required to build RNTester.".red
+    `brew install cmake`
+  else
+    Pod::UI.puts "In order to build RNTester locally, you need cmake installed, please install it and try again".red
+    return
+  end
+else
+  Pod::UI.puts "Cmake found at: #{cmake_path}".green
+end
+
+
 prepare_react_native_project!
 
 IN_CI = ENV['CI'] == 'true'