test: build testver in with the C90 standard

By building at least one test in C90 mode, we make sure the SDL headers are C90 compatible.

test/CMakeLists.txt

@@ -405,6 +405,8 @@ add_sdl_test_executable(testpalette SOURCES testpalette.c)
 add_sdl_test_executable(testtimer NONINTERACTIVE NONINTERACTIVE_ARGS --no-interactive NONINTERACTIVE_TIMEOUT 60 SOURCES testtimer.c)
 add_sdl_test_executable(testurl SOURCES testurl.c)
 add_sdl_test_executable(testver NONINTERACTIVE NOTRACKMEM SOURCES testver.c)
+set_property(TARGET testver PROPERTY C_STANDARD 90)
+set_property(TARGET testver PROPERTY C_EXTENSIONS OFF)
 add_sdl_test_executable(testcamera MAIN_CALLBACKS SOURCES testcamera.c)
 add_sdl_test_executable(testclipboard MAIN_CALLBACKS SOURCES testclipboard.c ${icon_png_header} DEPENDS generate-icon_png_header)
 add_sdl_test_executable(testviewport NEEDS_RESOURCES TESTUTILS SOURCES testviewport.c)