Fix install paths in libcmark.pc

[smancill]

CMAKE_INSTALL_<dir> can be relative or absolute path, so it is wrong to prefix CMAKE_INSTALL_PREFIX because if CMAKE_INSTALL_<dir> is set to an absolute path it will result in a malformed path with two absolute paths joined together:

$ cmake -DCMAKE_INSTALL_PREFIX=/foo/bar \
    -DCMAKE_INSTALL_BINDIR=/foo/bar/bin \
    -DCMAKE_INSTALL_LIBDIR=/foo/bar/lib \
    -DCMAKE_INSTALL_INCLUDEDIR=/foo/bar/include \
    ..

$ cat src/libcmark.pc
prefix=/foo/bar
exec_prefix=/foo/bar
libdir=/foo/bar//foo/bar/lib
includedir=/foo/bar//foo/bar/include
...

The libdir and includedir should be set by either calculating the directories relative to the prefix or by using the CMAKE_INSTALL_FULL_<dir> variables from GNUInstallDirs.

The CMAKE_INSTALL_FULL_<dir> variables seem the simpler solution.

---

The issue was observed with Nix package manager, which sets all CMAKE_INSTALL_<dir> variables to the full path of the Nix store when building the package:

$ nix-env -f'<unstable>' -iA cmark
$ cat /nix/store/q57a8pzqlfh0nfn0ncl5nxzksfshf1j8-cmark-0.30.1/lib/pkgconfig/libcmark.pc
prefix=/nix/store/q57a8pzqlfh0nfn0ncl5nxzksfshf1j8-cmark-0.30.1
exec_prefix=/nix/store/q57a8pzqlfh0nfn0ncl5nxzksfshf1j8-cmark-0.30.1
libdir=/nix/store/q57a8pzqlfh0nfn0ncl5nxzksfshf1j8-cmark-0.30.1//nix/store/q57a8pzqlfh0nfn0ncl5nxzksfshf1j8-cmark-0.30.1/lib
includedir=/nix/store/q57a8pzqlfh0nfn0ncl5nxzksfshf1j8-cmark-0.30.1//nix/store/q57a8pzqlfh0nfn0ncl5nxzksfshf1j8-cmark-0.30.1/include

Name: libcmark
Description: CommonMark parsing, rendering, and manipulation
Version: 0.30.1
Libs: -L${libdir} -lcmark
Cflags: -I${includedir}

[jgm]

@compnerd @nwellnhof - This seems right to me, but I wanted to check with you since I'm such an ignoramus about cmake.

[nwellnhof]

Looks good to me.

[jgm]

Thanks!