opus.pc variable substitution bug

There is a problem with variable substitution when generating the opus.pc pkg-config file using CMake.

CMakeLists.txt executes configure_file(opus.pc.in opus.pc), which according to the CMake docs will substitute occurrences of @VAR@ AND ${VAR} in opus.pc.in. See: https://cmake.org/cmake/help/v3.10/command/configure_file.html

However, since this is a .pc file which uses the ${VAR} syntax for its own substitution purposes, we really only want @VAR@ to be substituted. The problem is that ${libdir} and ${includedir} in the opus.pc.in file (shown below) are substituted when they shouldn't be. This results in absolute paths being embedded in -L and -l flags, breaking pkg-config's own substitution/relocation/prefix mechanism.

Libs: -L${libdir} -lopus
Cflags: -I${includedir}/opus

I fixed this locally by adding the @ONLY attribute: configure_file(opus.pc.in opus.pc @ONLY). The resulting opus.pc file is relocatable and works smoothly with the usual pkg-config --define-prefix mechanism.

Inspect CMakeLists.txt and opus.pc.in, starting with the configure_file call and the ${libdir} and ${includedir} entries. Verify that only CMake-style substitutions occur and that the generated opus.pc preserves pkg-config variables for relocation; confirm the result with the existing build and generated file.