Fix pkg-config File Generation Again

Fix pkg-config File Generation Again

Revises #1851. Fixes #1900. Replaces #1930.

Thanks to @orbea, @neheb, @Polynomial-C, and particularly @eli-schwartz for
pointing out the problem and suggesting solutions.

Tested with

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

  # 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

  # 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

  # 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
  ```