Use pkg-config in a cross-compiler-friendly way
Issue detected by Debian's QA tool "Lintian".
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configure: Use PKG_PROG_PKG_CONFIG instead of reinventing it
When cross-compiling (for example for aarch64 on x86), using AC_PATH_PROG to search for pkg-config will find the build architecture (x86) pkg-config, but what we want is the host architecture (aarch64) pkg-config. PKG_PROG_PKG_CONFIG does this correctly, by using AC_PATH_TOOL.
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configure: Run $PKG_CONFIG instead of hard-coding pkg-config