Expand search paths for find_library()

[kcgthb]

Don't restrict paths where find_library() will look for things, let it use CMAKE_PREFIX_PATH and CMAKE_LIBRARY_PATH when defined.

This is in addition of the paths defined with SET(SEARCH_PATH and won't prevent detection of locally installed libraries (via package managers).

See #27

[kcgthb]

For instance, if I load an OpenBLAS module that adds /share/software/user/open/openblas/0.3.20/lib to CMAKE_LIBRARY_PATH:

• without that change, cmake only detects local installations of OpenBLAS:

  -- Interface to Extra BLAS Libraries:
  --  /usr/lib64/libopenblas.so - OpenBLASSerial
  --  /usr/lib64/libopenblasp.so - OpenBLASPThread
  --  /usr/lib64/libopenblaso.so - OpenBLASOpenMP

• with the change, the new path is also detected:

  -- Interface to Extra BLAS Libraries:
  --  /share/software/user/open/openblas/0.3.20/lib/libopenblas.so - OpenBLASSerial
  --  /usr/lib64/libopenblasp.so - OpenBLASPThread
  --  /usr/lib64/libopenblaso.so - OpenBLASOpenMP