Remove some obsolete material from the build instructions.

It no longer works to create the "build" directory and then build
separate optimized and debug versions.  As confirmed in IRC with sm
and egh, the former instructions resulted in an error:

  $ mkdir build
  $ ./acprep opt make
  make: *** No targets specified and no makefile found.  Stop.
  acprep: ERROR: Execution failed: make
  $ ./acprep debug make
  make: *** No targets specified and no makefile found.  Stop.
  acprep: ERROR: Execution failed: make

Maybe there's a fix for this, but I don't know what it is, so this
commit just makes the build instructions work for the common case.
Merge pull request #210 from OpenTechStrategies/fix-build-instructions

Remove some obsolete material from the build instructions.
Guard against a possible null value
Update ledger-mode's "Quick Installation" docs

* Change quote character to '
* Fix indentation
* Load "ledger-mode" rather than "ldg-new"
delete obsolete ledger.texi. Down with cruft!
Merge pull request #211 from mneilsen/master

Update ledger-mode's "Quick Installation" docs
Fix reference to old pattern negation syntax
Merge pull request #212 from bentglasstube/master

Fix reference to old pattern negation syntax
fix README.md build command

--prefix= should not be passed to cmake directly. If the extra '--'
is removed then the prefix get passed correctly to cmake.
Merge pull request #213 from kain88-de/readme_typo

fix README.md build command