DOC: Add cmake flag for python virtual environments

I'm of the masechist variety and don't like anaconda coming between me
and python. I also like to run python virtual environments. When running
cmake, find_packages(Numpy) did not work because my python exe path was
not recognized. This flag helps cmake find it.

I've also added a build/ directory into the .gitignore path for those
who like to build within the repo. That's pretty standard practice for
cmake.

.gitignore

@@ -2,3 +2,4 @@
 *~
 extensions/isce/core/export/
 *.swp
+build/

doc/doxygen/install/centos.dox

@@ -209,6 +209,7 @@ Other optional arguments can be added to the cmake line
     <tr><td rowspan="3">-DMEMORY_CHECK_COMMAND=PATH_TO_VALGRIND_EXECUTABLE -DMEMORYCHECK_COMMAND_OPTIONS="--trace-children=yes --leak-check=full --track-origins=yes" -DCMAKE_BUILD_TYPE=Debug<td>  
     <tr><td>Run tests with "-T memcheck" to check for memory leaks.
     <tr><td>valgrind needs to be installed.
+    <tr><td>-DPYTHON_EXECUTABLE:FILENAME=<path_to_python_exe><td>Pass this argument if installing to a python virtual environment
 </table>
 
 <li> Build the software