Update docs

Author: petk

docs/cmake-modules/PHP/AddCustomCommand.md

@@ -0,0 +1,52 @@
+# PHP/AddCustomCommand
+
+See: [AddCustomCommand.cmake](https://github.com/petk/php-build-system/tree/master/cmake/cmake/modules/PHP/AddCustomCommand.cmake)
+
+Add custom command.
+
+This module is built on top of the CMake
+[`add_custom_command`](https://cmake.org/cmake/help/latest/command/add_custom_command.html)
+and [`add_custom_target()`](https://cmake.org/cmake/help/latest/command/add_custom_target.html)
+commands.
+
+A common issue in build systems is the generation of files with the project
+program itself. Here are two main cases:
+* PHP is found on the system: this is the most simple and developer-friendly to
+  use as some files can be generated during the build phase.
+* When PHP is not found on the system, ideally the files could be generated
+  after the PHP CLI binary is built in the current project itself. However, this
+  can quickly bring cyclic dependencies between the target at hand, PHP CLI and
+  the generated files. In such case, inconvenience is that two build steps might
+  need to be done in order to generate the entire project once the file has been
+  regenerated.
+
+This module exposes the following function:
+
+```cmake
+php_add_custom_command(
+  <unique-symbolic-target-name>
+  OUTPUT  ...
+  DEPENDS ...
+  PHP_COMMAND ...
+  [COMMENT <comment>]
+  [VERBATIM]
+)
+```
+
+It acts similar to `add_custom_command()` and `add_custom_target()`, except that
+the DEPENDS argument doesn't add dependencies among targets but instead checks
+their timestamps manually and executes the PHP_COMMAND only when needed.
+
+```cmake
+php_add_custom_command(
+  php_generate_something
+  OUTPUTS
+    list of generated files
+  DEPENDS
+    list of files or targets that this generation depends on
+  PHP_COMMAND
+    ${CMAKE_CURRENT_SOURCE_DIR}/generate-something.php
+  COMMENT "Generate something"
+  VERBATIM
+)
+```

docs/evolution.md

@@ -201,9 +201,6 @@ including the removal of `aclocal.m4` in favor of the php related M4 macros in
 
 * The hash extension is now always available, meaning the `--enable-hash`
   configure argument has been removed.
-* The filter extension no longer exposes the `--with-pcre-dir` configure
-  argument and therefore allows shared builds with `./configure` for Unix
-  builds.
 * Symbols `HAVE_DATE`, `HAVE_REFLECTION`, and `HAVE_SPL` have been removed. It
   should be considered to have these extensions always available.
 * Removed unused build time symbols: `PHP_ADA_INCLUDE`, `PHP_ADA_LFLAGS`,
@@ -233,6 +230,8 @@ including the removal of `aclocal.m4` in favor of the php related M4 macros in
 
 ##### Configure options
 
+* The filter extension no longer exposes the `--with-pcre-dir` configure
+  argument and therefore allows shared builds with `./configure`.
 * Added new `--enable-rtld-now` configure option to switch the dlopen behavior
   from `RTLD_LAZY` to `RTLD_NOW`.
 * The `--with-pcre-valgrind` and `--with-valgrind` were merged, and Valgrind