avformat may fail to be enabled, with no warning, when using --enable-avformat

[RedDwarf]

Using the latest snapshot tarball on Fedora 33 this happens:

$ ./configure --disable-all --enable-avformat --enable-demuxer=matroska
install prefix            /usr/local
source path               .
C compiler                gcc
C library                 glibc
ARCH                      x86 (generic)
big-endian                no
runtime cpu detection     yes
standalone assembly       yes
x86 assembler             nasm
MMX enabled               yes
MMXEXT enabled            yes
3DNow! enabled            yes
3DNow! extended enabled   yes
SSE enabled               yes
SSSE3 enabled             yes
AESNI enabled             yes
AVX enabled               yes
AVX2 enabled              yes
AVX-512 enabled           yes
XOP enabled               yes
FMA3 enabled              yes
FMA4 enabled              yes
i686 features enabled     yes
CMOV is fast              yes
EBX available             yes
EBP available             yes
debug symbols             yes
strip symbols             yes
optimize for size         no
optimizations             yes
static                    yes
shared                    no
postprocessing support    no
network support           no
threading support         pthreads
safe bitstream reader     yes
texi2html enabled         no
perl enabled              yes
pod2man enabled           yes
makeinfo enabled          no
makeinfo supports HTML    no

External libraries:
iconv                   zlib

External libraries providing hardware acceleration:
cuda_llvm               v4l2_m2m

Libraries:
avutil

Programs:

Enabled decoders:

Enabled encoders:

Enabled hwaccels:

Enabled parsers:

Enabled demuxers:
matroska

Enabled muxers:

Enabled protocols:

Enabled filters:

Enabled bsfs:

Enabled indevs:

Enabled outdevs:

License: LGPL version 2.1 or later

The configure script has this function

disable_with_reason(){
    disable $1
    eval "${1}_disable_reason=\"$2\""
    if requested $1; then
        die "ERROR: $1 requested, but $2"
    fi
}

called by check_deps() so I would expect at the very least a warning.

Failing to enable the library, with no message, and at the same time saying the matroska demuxer is enabled is at least confusing.

FWOW adding --enable-avcodec "solves" the issue, even without --enable-avutil.

[Carl Eugen Hoyos]

libavformat depends on libavcodec.

[RedDwarf]

Don't you think that if the user says "--enable-avformat" and the configure script, for whatever reason, doesn't enable it it should either:

• Fail with an error
• Succeed with a warning

?

Sure, the dependency exists. But the intention of a user writing "--disable-all --enable-avformat" is clear and the configure script is just ignoring him.