CMake: Improve module finding

[certik]

If Lapack is installed in a custom location, the only way to compile dftatom is via this hack:

$ CC=gcc CXX=g++ FC=gfortran cmake -DWITH_C_INTERFACE=yes -DWITH_LAPACK=yes . -DCMAKE_SHARED_LINKER_FLAGS="-L$HASHSTACK/lib -Wl,-rpath,$HASHSTACK/lib" -DCMAKE_EXE_LINKER_FLAGS="-L$HASHSTACK/lib -Wl,-rpath,$HASHSTACK/lib" -DCMAKE_MODULE_LINKER_FLAGS="-L$HASHSTACK/lib -Wl,-rpath,$HASHSTACK/lib"
-- The C compiler identification is GNU 4.9.1
-- The CXX compiler identification is GNU 4.9.1
-- Check for working C compiler: /home/certik/repos/hashstack/default/bin/gcc
-- Check for working C compiler: /home/certik/repos/hashstack/default/bin/gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /home/certik/repos/hashstack/default/bin/g++
-- Check for working CXX compiler: /home/certik/repos/hashstack/default/bin/g++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- The Fortran compiler identification is GNU
-- Check for working Fortran compiler: /home/certik/repos/hashstack/default/bin/gfortran
-- Check for working Fortran compiler: /home/certik/repos/hashstack/default/bin/gfortran  -- works
-- Detecting Fortran compiler ABI info
-- Detecting Fortran compiler ABI info - done
-- Checking whether /home/certik/repos/hashstack/default/bin/gfortran supports Fortran 90
-- Checking whether /home/certik/repos/hashstack/default/bin/gfortran supports Fortran 90 -- yes
-- Found Lapack: lapack;blas  


Configuration results
---------------------
Fortran compiler: /home/certik/repos/hashstack/default/bin/gfortran
Build type: Debug
Fortran compiler flags: -std=f2003 -Wall -Wextra -Wimplicit-interface -fPIC -fmax-errors=1 -g -fbounds-check -fcheck-array-temporaries -fbacktrace
Installation prefix: /usr/local
LIBS: lapack;blas
With C interface: yes
With Python: no
With Lapack: yes
LAPACK_LIBRARIES: lapack;blas
-- Configuring done
-- Generating done
-- Build files have been written to: /home/certik/repos/dftatom

(This also uses custom compiled gcc 4.9.1 via the line CC=gcc CXX=g++ FC=gfortran, but that's fine.)

The way to do that is to allow the user to specify custom location of dependent libraries. We have figured it out in CSymPy, e.g. here: https://github.com/sympy/csympy/blob/96e80f5b9f67e5f02cbfaa7a5b941e64193b5455/cmake/FindGMP.cmake, so we can just copy that approach.