These compilers pass the inline assembly check, but fail to compile
our inline assembly in various ways.

Otherwise inline assembly may get enabled when disabled on the command line.

Deprecated in 11/2013.

JACK is not commonly installed and should not be picked up as a
dependency unless specifically requested.

Jack uses weak symbols on purpose.

Recent versions of macOS provide more POSIX API (in particular,
clock_gettime) than previous versions and recent Apple toolchains
provide all that API, even when targeting older releases without
said API. Disallow linking to functions which might not be available
at runtime.

To actually have an effect, either add
--extra-cflags="-mmacosx-version-min=10.11" (or any other version
prior to 10.12) or set MACOSX_DEPLOYMENT_TARGET=10.11 when running
configure.

As a workaround for libav versions without this fix, one can
also add --extra-cflags="-mmacosx-version-min=10.11
-Werror=partial-availability" while running configure.

The -no_weak_imports flag is new in Xcode 8; in Xcode 7 it is not
supported. This is not an issue since Xcode 7 only ships with the
10.11 macOS SDK, which lacks clock_gettime.

Bug-Id: 1033

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

NASM is more actively maintained and permits generating dependency information
as a sideeffect of assembling, thus cutting build times in half.

SDL adds some "special" CFLAGS that interfere with building normal
binaries. Capture those CFLAGS separately and avoid adding them to
the general CFLAGS.

We should only check for clock_gettime() if _POSIX_MONOTONIC_CLOCK is
available and do a full link check for clock_gettime() in all cases.

Consistently use object format names with "32" suffix and set object format
to "win64" on Windows x86_64, which fixes assembling with nasm.

libavutil uses pthreads in the buffer code (abstracted through a header).

None of them are specific to the YASM assembler.

Also drop stray duplicate OBJCC config.mak entry.

Previously, all link-time dependencies were added for all libraries,
resulting in bogus link-time dependencies since not all dependencies
are shared across libraries. Also, in some cases like libavutil, not
all dependencies were taken into account, resulting in some cases of
underlinking.

To address all this mess a machinery is added for tracking which
dependency belongs to which library component and then leveraged
to determine correct dependencies for all individual libraries.

Leaving those variables in an undefined state allows them getting implicitly
enabled when they are declared as weak dependencies of other components.
In that case, the library check is not run and required linker flags are not
added, resulting in a failing build.

Fixes linking when enabling libfreetype without libfontconfig.

Have check_pkg_config() enable variables and set cflags and extralibs
instead of relegating that task to require_pkg_config. This simplifies
require_pkg_config(), is consistent with what other helper functions
like check_lib() do and allows getting rid of some manual variable
setting in places where check_pkg_config() is used.

This is less confusing than encountering "" in the argument list.

Newer versions of libxcb have xcb-foo pkg-config files that do not declare
their xcb dependency so that required linker flags will not be generated.

The early check for inconsistent in-source vs out-of-source build
cannot generate a config.log otherwise.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

Fixes all sorts of configuration problems introducec by dad7a9c7
on non-Linux or non-vanilla configs. Also removes a line made redundant
in that commit.

This makes the feature more visible and obvious.

This allows distinguishing between the internal variable name for
external libraries and the pkg-config package name. Having both
names available avoids special-casing outside the helper function
when the two identifiers do not match.

This ensures that added CPPFLAGS are validated against libc headers.

Simplifies checking for conditions in external library headers and
aborting if said conditions are not met.

Simplifies checking for external library headers and aborting if
the external library support was requested, but is not available.

Weak dependencies on external libraries do not obviate having to
explicitly enable these libraries, so the weak dependency does not
simplify the configure command line nor have any real effect.