configure.in: cause a reasonable diagnostic if AX_CHECK_COMPILE_FLAG is not available

[RossBencina]

On some platforms the AX_CHECK_COMPILE_FLAG macro (used in our configure.in) is not defined within the base autotools package, but rather in a separate autotools-archive package. If that package is not installed AX_CHECK_COMPILE_FLAG is not defined. This results in the use of AX_CHECK_COMPILE_FLAG being emitted verbatim into ./configure, which in turn results in a cryptic error message:

./configure: line 19323: syntax error near unexpected token `-std=c11,'
./configure: line 19323: `        AX_CHECK_COMPILE_FLAG(-std=c11, CFLAGS="-std=c11 $CFLAGS", CFLAGS="-std=c99 $CFLAGS")'
Error: Process completed with exit code 2.

This patch improves the error diagnostic. The new diagnostic will be emitted at the time that autoreconf is run, and will not allow the ./configure file to be generated if the AX_CHECK_COMPILE_FLAG macro is not defined. The new error is:

configure.ac:10: error: possibly undefined macro: AX_CHECK_COMPILE_FLAG
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
autoreconf-2.72: error: /usr/bin/autoconf-2.72 failed with exit status: 1

[philburk]

The "dnl" at the end seems unnecessary.