require_pkg_config() dies with useless error message when pkg-config is missing

[Actium]

Summary of the bug:

Enable an external library that is ./configure'd via require_pkg_config(), e.g. opus, with pkg-config not being installed. ./configure dies complaining that the library was "not found", although it is properly installed (after all everything works fine if pkg-config is installed). This error message is unhelpful if not misleading.

Actually an appropriate error message gets generated

configure:2913:
if ! $pkg_config --version >/dev/null 2>&1; then
    warn "$pkg_config not found, library detection may fail."
    pkg_config=false
fi

but since warn() defers the output of warnings and require_pkg_config() dies too early

configure:1158: check_pkg_config "$@" || die "ERROR: $pkg not found"

the warning is only placed in config.log, but ~1.5k lines away from the "ERROR: $pkg not found" and thus quite difficult to stumble upon (I only found the warning after I started taking apart the configure script).

How to reproduce:

# aptitude purge pkg-config
$ git clone --depth 1 git://source.ffmpeg.org/ffmpeg.git ffmpeg ; cd ffmpeg
$ ./configure --enable-libopus
ERROR: opus not found

If you think configure made a mistake, make sure [...]

Suggested fix

Since $pkg_config is set to false if pkg-config is not available

configure:2914:
if ! $pkg_config --version >/dev/null 2>&1; then
    warn "$pkg_config not found, library detection may fail."
    pkg_config=false
fi

require_pkg_config() could easily be patched to die with a meaningful error message in that case (see attached patch).

Alternatively die() could be modified to output deferred warnings or warn() could print messages immediately.