Create FONTFORGE_ARG_WITHOUT()

Based on existing code example from FONTFORGE_ARG_WITH_LIBREADLINE()
Used to reduce repeated code, reduce possible errors.
Expands to introduce ___CFLAGS, ___LIBS, and ask/test --without-___
Create FONTFORGE_BUILD_YES_NO_HALT()

Based on existing code example from FONTFORGE_ARG_WITH_LIBSPIRO()
Used to reduce repeated code, reduce possible errors, and mainly cause
a forced-halt if users specify --with but don't have the required
development libraries required to compile with.
Import CFLAGS and LIBS tests from FONTFORGE_ARG_WITH_LIBREADLINE()
Skip test for pkg-config. test for gif_lib.h and giflib or libungif.

Some distros are not completely configured correctly for pkg-config.
There is a school of thought that argues for using PKG_CHECK_MODULES,
but that is not necessary. The standard way to check if a library
exists and to link against it is to use AC_CHECK_LIB(), and also check
for the developer header file(s) using AC_CHECK_HEADERS().

Fixed fontforge_create_pkg-config.m4 to only include giflib or libungif
as a dependecy ONLY if it exists AND user didn't say no to include it.

This comes to a hard-stop if user wants giflib, but it's not available.
Merge pull request #1415 from JoesCat/check_headers_and_libraries_2

Check headers and libraries 2