Fix pkg-config File Generation Again Again

Fix pkg-config File Generation Again Again

Resubmission of #2001. This switches the `sed` invocations to use `-E`,
extended regex syntax, which is better standardized across platforms.
I guess.

Same test plan:

```
make -C lib clean libzstd.pc
cat lib/libzstd.pc

echo # should fail
make -C lib clean libzstd.pc     LIBDIR=/foo
make -C lib clean libzstd.pc INCLUDEDIR=/foo
make -C lib clean libzstd.pc     LIBDIR=/usr/localfoo
make -C lib clean libzstd.pc INCLUDEDIR=/usr/localfoo
make -C lib clean libzstd.pc     LIBDIR=/usr/local/lib     prefix=/foo
make -C lib clean libzstd.pc INCLUDEDIR=/usr/local/include prefix=/foo

echo # should succeed
make -C lib clean libzstd.pc     LIBDIR=/usr/local/foo
make -C lib clean libzstd.pc INCLUDEDIR=/usr/local/foo
make -C lib clean libzstd.pc     LIBDIR=/usr/local/
make -C lib clean libzstd.pc INCLUDEDIR=/usr/local/
make -C lib clean libzstd.pc     LIBDIR=/usr/local
make -C lib clean libzstd.pc INCLUDEDIR=/usr/local
make -C lib clean libzstd.pc     LIBDIR=/tmp/foo prefix=/tmp
make -C lib clean libzstd.pc INCLUDEDIR=/tmp/foo prefix=/tmp
make -C lib clean libzstd.pc     LIBDIR=/tmp/foo prefix=/tmp/foo
make -C lib clean libzstd.pc INCLUDEDIR=/tmp/foo prefix=/tmp/foo

echo # should also succeed
make -C lib clean libzstd.pc prefix=/foo LIBDIR=/foo/bar INCLUDEDIR=/foo/
cat lib/libzstd.pc

mkdir out
cd out
cmake ../build/cmake
make
cat lib/libzstd.pc
```