Provide deltachat/deltachat.h in builddir

This makes sure that there is a deltachat/deltacht.h file in the
build directory before it is installed.  This allows code to
to #include <deltachat/deltachat.h> when building against an
uninstalled library just like they would for building against an
installed library.

deltachat/meson.build

@@ -0,0 +1,8 @@
+# This source directory only exists so we can have a
+# deltachat/deltachat.h in the build directory.  This allows C code to
+# always `#include <deltachat/deltachat.h>` even to build against an
+# uninstalled library by -I to point at the build directory.
+configure_file(input: '../src/deltachat.h',
+               output: 'deltachat.h',
+               copy: true,
+               install: false)

meson.build

@@ -128,6 +128,8 @@ else
   netpgp = netpgp_proj.get_variable('dep')
 endif
 
+# Provide deltachat/deltachat.h in the build directory
+subdir('deltachat')
 
 # Build the library, stored in `lib`.
 subdir('src')