Update the cmake find modules to follow FindGMP.cmake

But not exactly, I avoided using "${MODULE_DIR}/include" and
"${MODULE_DIR}/lib" in `find_path` and `find_library` respectively.
Enable user specified paths in `find_library` and `find_path`

Enable the user to specify paths to search for libraries and include files
when they are installed in non-standard locations.
Change `${MODULE_DIR}/include` to `${MODULE_DIR}/lib` in `find_library`
Macro to find packages in cmake
Fix macro to find flint, use macro in other cmake files
Merge branch 'master' into pr-314
FLINT: search for flint/flint.h

This is the recommended way in C/C++.
Add a workaround for ARB's handling of flint.h

This is just a temporary workaround until ARB is fixed.
Add documentation to LibFindMacros.cmake
Use *_INCLUDE_DIRS
Improve the documentation
Fix COMMON_DIR

Use as follows:

$ cmake -DWITH_ARB=yes -DCOMMON_DIR=$HASHSTACK

Where $HASHSTACK is the path to a Hashstack's profile. It can be any common
prefix for all installed libraries.
Rename libfind_path -> libfind_include
Merge pull request #315 from certik/cmake

Refactor the CMake files