[3.1 -> main] pinned_build.sh: make relative path work

[linh2931]

main branch merge of #179.
Resolve #130.

Two issues prevents relative paths from working:

1. if [[ ${D} != ${EXPECTED} ]]; then does string comparison. ../lib != /home/lib even though they refer to the same directory.
2. After the first problem was fixed, a second error occurred in cmake:

 The CMAKE_C_COMPILER:
    ../lib/clang-11.0.1/bin/clang
  is not a full path and was not found in the PATH.

After the fix,

./scripts/pinned_build.sh ../lib ./build 6 produces

...
CPackDeb: - Generating dependency list
 .----------------.  .----------------.  .----------------.  .----------------.
| .--------------. || .--------------. || .--------------. || .--------------. |
| |   _____      | || |  _________   | || |      __      | || |   ______     | |
| |  |_   _|     | || | |_   ___  |  | || |     /  \     | || |  |_   __ \   | |
| |    | |       | || |   | |_  \_|  | || |    / /\ \    | || |    | |__) |  | |
| |    | |   _   | || |   |  _|  _   | || |   / ____ \   | || |    |  ___/   | |
| |   _| |__/ |  | || |  _| |___/ |  | || | _/ /    \ \_ | || |   _| |_      | |
| |  |________|  | || | |_________|  | || ||____|  |____|| || |  |_____|     | |
| |              | || |              | || |              | || |              | |
| '--------------' || '--------------' || '--------------' || '--------------' |
Leap has successfully built and constructed its packages.  You should be able to find the packages at ./build.  Enjoy!!!