build.sh: another pass at the env variable help text

build.sh: another pass at the env variable help text

Thanks to prior work, this opens the door to another attempt
at improving the help text. Even until now, I was confused at what
some of the variables were used for.

These are the organizing principles I used in the hope of helping
new contributors use build.sh and long time users who may have
overlooked some of the available features.

1- The variables are group by category, meaning the "owner" of the
variables.

2- The first line is the definition of the variable

3- The second line is a hint as to what build.sh does with it

4- Use the Autotools help text where it applies rather than inventing your own
------------------------------------------------------------------------------

Environment variables specific to build.sh:
  QUIET       Do not print messages saying which checks are being made
              Each module/components is invoked with --quite
  GITROOT     Source code repository path [git://anongit.freedesktop.org/git]
              Optional when using --clone to update source code before building
  CONFFLAGS   Configure options to pass to all Autoconf configure scripts
              Refer to 'configure --help' from any module/components
  LIBDIR      Path segment under $PREFIX for libraries (e.g., lib64) [lib]
              Used to build the font path, search libraries and packages
  FONTPATH    Path to fonts directories [$PREFIX/$LIBDIR/X11/fonts/misc/, ...]
              Picked-up by the xserver as a value for --with-default-font-path

Environment variables defined by the GNU Build System:
  DESTDIR     Path to the staging area where installed objects are relocated
  MAKE        The name of the make command [make]
  MAKEFLAGS:  Options to pass to all $(MAKE) invocations
  CC          C compiler command
  CFLAGS      C compiler flags
  LDFLAGS     linker flags, e.g. -L<lib dir> if you have libraries in a
              nonstandard directory <lib dir>
  CPPFLAGS    C/C++/Objective C preprocessor flags, e.g. -I<include dir> if
              you have headers in a nonstandard directory <include dir>
  CPP         C preprocessor

Environment variables defined by the shell:
  PATH        List of directories that the shell searches for commands
              $DESTDIR/$PREFIX/bin is prepended

Environment variables defined by the dynamic linker:
  LD_LIBRARY_PATH List directories that the linker searches for shared objects
                  $DESTDIR/$PREFIX/$LIBDIR is prepended

Environment variables defined by the pkg-config system:
  PKG_CONFIG_PATH List directories that pkg-config searches for libraries
                  $DESTDIR/$PREFIX/share/pkgconfig and
                  $DESTDIR/$PREFIX/$LIBDIR/pkgconfig are prepended

Reviewed-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>