#37 Port configure.in to find Tcl 8.6.16 version and modernize it

[Sergei Golovan]

Hi,

While building BLT 3.0 (HEAD from the Git repository), I've stumbled on two issues with the configure script:

1. configure crashes on checking whether DBL_EPSILON can be computed (gcc doesn't like to compile printf() without stdio.h included)
2. configure can't correctly determine the Tcl version (reports literally TCL_MAJOR_VERSION instead of 8, because it is defined in tcl.h as "# define TCL_MAJOR_VERSION 8" with a space between # and define).

Please, find the attached patch configure1.patch with fixes to both issues.

Also, configure2.patch modernizes configure.in as recommended by new autoconf. Hopefully, it might be useful too.