Does not build without feature test macro set externally

[irowebbn]

Describe the bug
There's a build dependency here with [cmake/sample_defs/mission_build_custom.cmake](https://github.com/nasa/cFE/blob/03166722cdde3f1b337742088e7137ebacf64734/cmake/sample_defs/mission_build_custom.cmake), which specifies a feature test macro. If a mission has a different and/or non-existent mission_build_custom.cmake, the compilation will fail for this target. It would be much better if the target specified its own requirements.

# The _XOPEN_SOURCE directive is required for glibc to enable conformance with the
# the X/Open standard version 6, which includes POSIX.1c as well as SUSv2/UNIX98 extensions.
add_definitions(
    -D_XOPEN_SOURCE=600
)

To Reproduce

• Follow normal build instructions, but omit mission_build_custom.cmake from the <mission>_defs folder.

[ 50%] Building C object tools/elf2cfetbl/CMakeFiles/elf2cfetbl.dir/elf2cfetbl.c.o
make[4]: Leaving directory `build'
[ 50%] Built target cfe-build-env
inside get_version for core_api
tools/elf2cfetbl/elf2cfetbl.c:99:18: error: ‘PATH_MAX’ undeclared here (not in a function)
 char SrcFilename[PATH_MAX] = {""};
                  ^
tools/elf2cfetbl/elf2cfetbl.c: In function ‘OpenSrcFile’:
tools/elf2cfetbl/elf2cfetbl.c:1427:13: warning: implicit declaration of function ‘ctime_r’ [-Wimplicit-function-declaration]
             printf("Original Source File Modification Time: %s\n", ctime_r(&SrcFileStats.st_mtime, TimeBuff));
             ^
make[4]: *** [tools/elf2cfetbl/CMakeFiles/elf2cfetbl.dir/elf2cfetbl.c.o] Error 1

Expected behavior
The target builds successfully.

System observed on:

• OS: CentOS 7
• cFE/OSAL/PSP: draco-rc3 bundle

Reporter Info
Isaac Rowe, NASA JSC, Jacobs Technology