[rtlsdr] Fix the 'rtlsdr' package when not running from within a git tree

[BillyONeal]

No description provided.

[BillyONeal]

/cc https://github.com/echesakov

[BillyONeal]

@dan-shaw The quotes are necessary because the non-quoted version uses the variable name itself, rather than empty string, if the variable is undefined, so I'm going to go back to the original change.

C:\Users\billy\Desktop\demo>type CMakeLists.txt
cmake_minimum_required(VERSION 3.16)
project(demo LANGUAGES)

if (UNDEFINED_VARIABLE STREQUAL "")
set(RESULT_VARIABLE "true")
else()
set(RESULT_VARIABLE "false")
endif()

message(FATAL_ERROR "Result: ${RESULT_VARIABLE}")

C:\Users\billy\Desktop\demo>cmake -G Ninja -S . -B undefined
CMake Error at CMakeLists.txt:10 (message):
Result: false


-- Configuring incomplete, errors occurred!
See also "C:/Users/billy/Desktop/demo/undefined/CMakeFiles/CMakeOutput.log".

C:\Users\billy\Desktop\demo>cmake -G Ninja -DUNDEFINED_VARIABLE="" -S . -B defined_empty
CMake Error at CMakeLists.txt:10 (message):
Result: true


-- Configuring incomplete, errors occurred!
See also "C:/Users/billy/Desktop/demo/defined_empty/CMakeFiles/CMakeOutput.log".

C:\Users\billy\Desktop\demo>