Add RPM packaging support (#5177)

* Add RPM packaging support

Using the existing PackageDebian as template add support for
generating an rpm with the package target.

* Restore debian package target

Also add an option to advertise the fact.

* Update package description

C-n-p from README.md

* Update rpm package maintainer

CMakeLists.txt

@@ -23,7 +23,13 @@ option(FLATBUFFERS_LIBCXX_WITH_CLANG "Force libc++ when using Clang" ON)
 option(FLATBUFFERS_CODE_SANITIZE
       "Add '-fsanitize' flags to 'flattests' and 'flatc' targets."
       OFF)
-
+option(FLATBUFFERS_PACKAGE_REDHAT
+       "Build an rpm using the 'package' target."
+       OFF)
+option(FLATBUFFERS_PACKAGE_DEBIAN
+       "Build an deb using the 'package' target."
+       OFF)
+  
 if(NOT FLATBUFFERS_BUILD_FLATC AND FLATBUFFERS_BUILD_TESTS)
     message(WARNING
     "Cannot build tests without building the compiler. Tests will be disabled.")
@@ -443,6 +449,13 @@ endif()
 
 include(CMake/BuildFlatBuffers.cmake)
 
-if(FLATBUFFERS_PACKAGE_DEBIAN)
-    include(CMake/PackageDebian.cmake)
+if(UNIX)
+    # Use of CPack only supported on Linux systems.
+    if(FLATBUFFERS_PACKAGE_DEBIAN)
+        include(CMake/PackageDebian.cmake)
+    endif()
+    if (FLATBUFFERS_PACKAGE_REDHAT)
+        include(CMake/PackageRedhat.cmake)
+    endif()
+    include(CPack)
 endif()