cmake: configure libsecp256k1.pc during install #1692

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.

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

cc @hebasto

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:

$ cmake -B build
$ cmake --build build
$ install_dir=$(mktemp -d)
$ cmake --install build --prefix $install_dir
$ cat ${install_dir}/lib/pkgconfig/libsecp256k1.pc 
prefix=/usr/local
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/include

Name: libsecp256k1
Description: Optimized C library for EC operations on curve secp256k1
URL: https://github.com/bitcoin-core/secp256k1
Version: 0.6.1
Cflags: -I${includedir}
Libs: -L${libdir} -lsecp256k1

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

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.

ACK 7106dce6fd410043fc297139c6307cb6016d2a7b I verified that it fixes the path in libsecp256k1.pc

While working on this issue earlier, I also considered support for relative paths.

I'm not sure how important this case is, but the current solution does not handle relative paths correctly. Consider the following examples:

• using CMAKE_INSTALL_PREFIX during configuration:

$ cmake -B build -DCMAKE_INSTALL_PREFIX=../install/a
$ cmake --build build
$ cmake --install build
$ cat ../install/a/lib/pkgconfig/libsecp256k1.pc | head -1
prefix=/home/hebasto/dev/secp256k1/install/a

• using the --prefix option during installation:

$ cmake -B build
$ cmake --build build
$ cmake --install build --prefix ../install/b
$ cat ../install/b/lib/pkgconfig/libsecp256k1.pc | head -1
prefix=../install/b

@real-or-random, what @hebasto shows is a severe regression for the use case of creating packages. When building binary packages for distribution, you usually install to a temporary directory and then create a tarball of that directory. Since pkg-config does not support relocatable packages, the .pc file needs to contain the location where the tarball will be extracted to, not the location that cmake installs to. The builtin default of /usr/local or the value of CMAKE_INSTALL_PREFIX is actually more correct than the argument to cmake --install.

Reverted in #1694.