cmake: configure libsecp256k1.pc during install #1692

1. 
   purpleKarrot commented at 5:10 pm on June 27, 2025: contributor

   When installing to a given prefix, make sure that the .pc file contains that prefix rather than the value of CMAKE_INSTALL_PREFIX that the project was configured with.
2. cmake: configure libsecp256k1.pc during install

   When installing to a given prefix, make sure that the .pc file contains
   that prefix rather than the value of CMAKE_INSTALL_PREFIX that the
   project was configured with.

   7106dce6fd
3. real-or-random added the label bug on Jun 27, 2025
4. real-or-random added the label build on Jun 27, 2025
5. 
   real-or-random commented at 6:29 pm on June 27, 2025: contributor

   Thanks! Can you provide instructions for a test case that is fixed by this?

   cc @hebasto

6. 
   hebasto commented at 7:45 pm on June 27, 2025: member

   cc @hebasto

   I haven’t tested this PR yet, but the issue I reported to @purpleKarrot offline can be reproduced on the master branch as follows:

    0$ cmake -B build
    1$ cmake --build build
    2$ install_dir=$(mktemp -d)
    3$ cmake --install build --prefix $install_dir
    4$ cat ${install_dir}/lib/pkgconfig/libsecp256k1.pc 
    5prefix=/usr/local
    6exec_prefix=${prefix}
    7libdir=${exec_prefix}/lib
    8includedir=${prefix}/include
    9
   10Name: libsecp256k1
   11Description: Optimized C library for EC operations on curve secp256k1
   12URL: https://github.com/bitcoin-core/secp256k1
   13Version: 0.6.1
   14Cflags: -I${includedir}
   15Libs: -L${libdir} -lsecp256k1
   

   Note prefix=/usr/local, which is obviously incorrect.

7. 
   purpleKarrot commented at 8:47 pm on June 27, 2025: contributor

   What @hebasto wrote.

   Also, when testing this, try installing as root and then verify whether you can delete the build directory with non-root permission. Also, try installing twice in a row and verify that the second install does not modify the timestamp of the file.

Contributors
purpleKarrot real-or-random hebasto

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