title: fatal error: Eigen/Dense: No such file or directory
body: <p>I have installed <code>libeigen3-dev</code> in order to compile programs using Eigen 3. when I include a file, such as <code>Eigen/Dense</code> I get this error when I try to run <code>g++</code>:</p>
  
  <pre><code>user@office-debian:~/Documents/prog$ g++ src/main.cpp -MMD -std=c++11
  In file included from src/main.cpp:9:0:
  src/tdefs.h:16:23: fatal error: Eigen/Dense: No such file or directory
  compilation terminated.
  </code></pre>
  
  <p>Running the following line works fine:</p>
  
  <blockquote>
    <p>g++ -I /usr/include/eigen3/ src/main.cpp -MMD -std=c++11</p>
  </blockquote>
  
  <p>shouldn't that include directory be automatically found by GCC because I installed the Eigen package through aptitude? Why are boost and OpenGL found automatically when I install the libraries but not Eigen? (Note that eigen is a header-only library, but that shouldn't matter right?)</p>
  
  <p>Running <code>g++ src/main.cpp -MMD -std=c++11 --verbose</code> produces the following output:</p>
  
  <pre><code>Using built-in specs.
  COLLECT_GCC=g++
  COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.7/lto-wrapper
  Target: x86_64-linux-gnu
  Configured with: ../src/configure -v --with-pkgversion='Debian 4.7.2-5' --with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs --enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.7 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
  Thread model: posix
  gcc version 4.7.2 (Debian 4.7.2-5) 
  COLLECT_GCC_OPTIONS='-MMD' '-std=c++11' '-v' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
   /usr/lib/gcc/x86_64-linux-gnu/4.7/cc1plus -quiet -v -imultiarch x86_64-linux-gnu -MMD main.d -D_GNU_SOURCE src/main.cpp -quiet -dumpbase main.cpp -mtune=generic -march=x86-64 -auxbase main -std=c++11 -version -o /tmp/ccoYRPKY.s
  GNU C++ (Debian 4.7.2-5) version 4.7.2 (x86_64-linux-gnu)
      compiled by GNU C version 4.7.2, GMP version 5.0.5, MPFR version 3.1.0-p10, MPC version 0.9
  GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
  ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
  ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../x86_64-linux-gnu/include"
  #include "..." search starts here:
  #include &lt;...&gt; search starts here:
   /usr/include/c++/4.7
   /usr/include/c++/4.7/x86_64-linux-gnu
   /usr/include/c++/4.7/backward
   /usr/lib/gcc/x86_64-linux-gnu/4.7/include
   /usr/local/include
   /usr/lib/gcc/x86_64-linux-gnu/4.7/include-fixed
   /usr/include/x86_64-linux-gnu
   /usr/include
  End of search list.
  GNU C++ (Debian 4.7.2-5) version 4.7.2 (x86_64-linux-gnu)
      compiled by GNU C version 4.7.2, GMP version 5.0.5, MPFR version 3.1.0-p10, MPC version 0.9
  GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
  Compiler executable checksum: 66d178dd81da8c975e003e06d9f5e782
  In file included from src/main.cpp:9:0:
  src/tdefs.h:16:23: fatal error: Eigen/Dense: No such file or directory
  compilation terminated.
  </code></pre>