Use PKG_PROG_PKG_CONFIG macro from pkg.m4 to detect pkg-config.

[sebastic]

The AC_PATH_PROG macro fails to select the correct version to support cross-compilation.

This triggers a lintian issue for the Debian package builds: ​autotools-pkg-config-macro-not-cross-compilation-safe

The package appears to use AC_PATH_PROG to discover the location of pkg-config(1). This macro fails to select the correct version to support cross-compilation.

A better way would be to use the PKG_PROG_PKG_CONFIG macro from pkg.m4 and then using the $PKG_CONFIG shell variable.

Refer to ​https://bugs.debian.org/884798 for details.

The attached patch implements this suggestion.

[strk]

In 16324:

Use PKG_PROG_PKG_CONFIG macro from pkg.m4 to detect pkg-config

Patch by Bas Couwenberg
Closes #3992

[strk]

In 16325:

Use PKG_PROG_PKG_CONFIG macro from pkg.m4 to detect pkg-config

Patch by Bas Couwenberg
Closes #3992 in 2.4 branch (for 2.4.4)

[strk]

In 16335:

Forgot to add pkg.m4 macro in r16324

See #3992

[strk]

In 16336:

Forgot to add pkg.m4 macro in r16324

See #3992