README.md: add build instructions

Author: multun

README.md

@@ -31,3 +31,45 @@ sh$ diff recovered-data.txt hidden-data.txt
 sh$ echo $?
 0
 ```
+
+# Build instructions
+
+The build process is a bit awkward, as pdfid is an in-tree patch of a library.
+
+The package depends on all packages needed by PoDoFo, plus `libgmp-dev`.
+
+```
+ cmake
+ libboost-dev
+ libcppunit-dev
+ libfontconfig1-dev
+ libfreetype6-dev
+ libidn11-dev
+ libjpeg-dev
+ liblua5.1-0-dev
+ libssl-dev
+ libtiff-dev
+ libunistring-dev
+ zlib1g-dev
+ libgmp-dev
+```
+
+``` sh
+mkdir build
+cd build
+
+# prepare the out of tree build.
+# the library must be staticaly linked against pdfid, as the shared
+# library would conflict with the unpatched version of PoDoFo.
+cmake -DPODOFO_BUILD_SHARED=OFF \
+      -DPODOFO_BUILD_STATIC=ON \
+      -DCMAKE_INSTALL_PREFIX=/usr \
+      -DCMAKE_BUILD_TYPE=Release ..
+
+# only build what's required.
+# the `all` target build all tests and tools
+make pdfid
+
+# run the correct subset of `make install`
+DESTDIR="${DESTDIR:?missing DESTDIR}" cmake -DCOMPONENT=pdfid -P ./cmake_install.cmake
+```

tools/pdfid/CMakeLists.txt

@@ -5,4 +5,5 @@ SET_TARGET_PROPERTIES(pdfid PROPERTIES COMPILE_FLAGS "${PODOFO_CFLAGS}")
 ADD_DEPENDENCIES(pdfid ${PODOFO_DEPEND_TARGET})
 INCLUDE_DIRECTORIES(${PoDoFo_SOURCE_DIR})
 INSTALL(TARGETS pdfid
+	COMPONENT pdfid
 	RUNTIME DESTINATION "bin")