Partially based on a patch by Timo Rothenpieler <timo@rothenpieler.org>.
Additional scaling list handling fix by Jun Zhao <mypopydev@gmail.com>.

This allows enabling or disabling the library-related variables from
within the function instead of doing it manually outside of it.

Check for and link against user32 instead, which also fixes
the missing dependency of dxva2 on user32 with MSVC.

Without any optimization flags, MSVC does no dead code elimination (DCE) at
all, even for the most trivial cases. DCE is a prerequisite for building libav
correctly, otherwise there are undefined references to functions for other
architectures and disabled components.

-O1 is the minimal optimization flag for MSVC that does include DCE.

Significantly increases the efficiency of frame threading, since
individual frames in a superframe can now be decoded in parallel.

This is required to make certain math defines visible on modern Cygwin.

This makes naming more consistent and simplifies extralibs-related changes.

This is more in line with the other related variable names.

These components should depend on the availability of the respective
libraries, not just on the availability of the respective headers.

These are taken care of more correctly by the library checks.

This is the standard behavior for external libraries.

This allows expressing dependencies in a more correct way.

This makes sense and takes care of missing build dependencies.

-pie was added to C flags for ThreadSanitizer in commit
19f251a288. Under clang 3.8.0, the -pie
flag causes a compiler warning and a linker error when running configure
--toolchain=clang-tsan. Here is an excerpt from config.log:

clang ... -fsanitize=thread -pie -std=c11 -fomit-frame-pointer -pthread -c -o /tmp/ffconf.A8SsaoCF.o /tmp/ffconf.JdpujQlD.c
clang: warning: argument unused during compilation: '-pie'
clang -fsanitize=thread -pie -Wl,--as-needed -o /tmp/ffconf.2iYA4bsw /tmp/ffconf.A8SsaoCF.o -lm -lm -lbz2 -lz -pthread
/usr/bin/ld: /tmp/ffconf.A8SsaoCF.o: relocation R_X86_64_PC32 against undefined symbol `atan2f@@GLIBC_2.2.5' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Bad value
clang: error: linker command failed with exit code 1 (use -v to see invocation)

To be conservative, I changed -pie to -fPIE. But the documentation seems
to imply just -fsanitize=thread is enough:

http://clang.llvm.org/docs/ThreadSanitizer.html
https://github.com/google/sanitizers/wiki/ThreadSanitizerCppManual

Signed-off-by: Wan-Teh Chang <wtc@google.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

The generic parameter names used for CFLAGS in configure must be filtered
for each compiler and replaced by the equivalent flag for that compiler.

This simplifies the code.

This ensures that dependencies are resolved correctly. COMPONENT_LIST
can contain parts that depend on previous entries of CONFIG_LIST.

Handle extralibs in the standard way, add missing pthreads dependency.
Also globally check for -fobj-arc with Objective-C compilers since
that option is useful for other Objective-C code as well.

The unadorned package name is needed to derive package-related variable names.

Fixes standalone compilation of the libmp3lame encoder.

It is of doubtful utility, adds complexity and no known users exist.

Supporting the system was a nice joke for the 9 release, but it has
run its course. Nowadays Plan 9 receives no testing and has no
practical usefulness.

The libdc1394 API transition was finished close to a decade ago.

This allows filtering out some warning noise.

Also drop -Winline from set of warning flags as it only produces noise.

Be explicit about which options should be set.

GnuTLS in combination with gcrypt has been deprecated since 2010.

Also add a missing avcodec.h #include in the movie filter.

This disables warnings about potentially uninitialized local pointer
variables.  Disabling the warning is in line with what we do for gcc.

The code now does not depend on VA and will work on windows as well.