- Use CMAKE_CXX_STANDARD
 - Use PROJET_NAME
 - Set PROJECT_VERSION
 - Use CMake search paths to find ExternalProject

The recommended method is to explicitly list all sources files for
targets. This way build configuration only needs to change if
CMakeFiles.txt is explicitly modified. There's no confusion about what
files are part of the build.

Each user configurable setting should have a corresponding option()
for booleans or typed cache variable for strings and paths.

The original library type was unclear, but POSITION_INDEPENDENT_CODE
would hint that shared library was intended. CMake automatically
enables PIC for shared libraries and modules.

The package creates an imported target Threads::Threads that
configures the correct library.

The test directory doesn't need to be a project, in fact it SHOULDN'T
be a project. All necessary flags etc. should be set by the library
target the tests link against.

ExternalProject used the most recent version of gtest which will
eventually lead to failing tests. Pin the version using a submodule.

Adding SOVERSION property to the target makes CMake generate symlinks
during installation. The RESOURCE property is supported by
install(TARGETS) and can be used to automatically install *.pc
files. The install(TARGETS) call specifies an EXPORT target name,
initialized to PROJECT_NAME, that can be used when adding this
repository as a submodule:

set(EXPORT_TARGET_NAME myproj)
add_subdirectory(tacopie)
install(TARGETS myexe EXPORT myproj
  RUNTIME)
install(EXPORT myproj DESTINATION lib/myproj)

The last line will generate ${PREFIX}/lib/myproj/myproj.cmake that
contains necessary CMake spells to include the installed files in
another projects as CMake targets. Somewhat like pkg-config, but nicer
for Windows people.