It's better to determine __STDC_VERSION__ by build system

[leleliu008]

I noticed that #319 introduced a variable JAS_STDC_VERSION to allow user set a proper value via -DJAS_STDC_VERSION=<VALUE> when cross compiling. the reason is try_run command can not run when cross compiling.

Actually, we do not need to compile a C source code to get the value of __STDC_VERSION__, we just need to run C Preprocesser like following shell code:

cc -std=c11 -E -dM - < /dev/null | sed -n '/__STDC_VERSION__/p' | cut -d ' ' -f3

translate above shell code to cmake script is as follows:

set(STDC_VERSION 0)

execute_process(
    COMMAND ${CMAKE_C_COMPILER} -std=c${CMAKE_C_STANDARD} -E -dM test.c
    OUTPUT_VARIABLE STDC_VERSION
    OUTPUT_STRIP_TRAILING_WHITESPACE
    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
)

string(REGEX MATCH "#define __STDC_VERSION__ [0-9]+L" STDC_VERSION "${STDC_VERSION}")
string(REPLACE " " ";" STDC_VERSION "${STDC_VERSION}")
list(GET STDC_VERSION 2 STDC_VERSION)

message("STDC_VERSION=${STDC_VERSION}")

the value of __STDC_VERSION__ is determined by -std=, -std= is fixed in CMakeLists.txt, so this value should be calculated by our build system, not by user.

[mdadams]

You cannot assume that JasPer is being built on a Linux/Unix platform. Also, you cannot assume that the compiler has a command-line interface similar to GCC/Clang. Many users of JasPer do not use Linux/Unix and/or GCC/Clang. So, "-std=..." will not have the desired effect in many cases. The same goes for the "-E", "-dM", and such.