The `PKG_CHECK_MODULES` function searches a pkg-config module and
then proceeds to set various variables containing information on
how to link to the library. In contrast to the `FIND_PACKAGE`
function, the library path set by `PKG_CHECK_MODULES` will not
necessarily contain linking instructions with a complete path to
the library, though. So when a library is not installed in a
standard location, the linker might later fail due to being
unable to locate it.

While we already honor this when configuring libssh2 by adding
`LIBSSH2_LIBRARY_DIRS` to the link directories, we fail to do so
for libcurl, preventing us to build libgit2 on e.g. FreeBSD. Fix
the issue by adding the curl library directory to the linker
search path.