Fix build on fedora 32, and don't enable define-prefix in pkg-config on Linux

commit 9e6bc136:

libsrtp.recipe: Fix build on Fedora 32 with GCC 10

commit 9302d179:

libunwind.recipe: Fix build on Fedora 32

commit c3f70153:

pkg-config.recipe: Do not enable --define-prefix on Linux
It doesn't work well if your prefix is not `usr/libblah/pkgconfig`,
such as on Debian.