The package export file allows CMake consumers to find Squirrel by
calling find_package(squirrel). This will create imported targets
for them to link against.
Create export files in both the build directory and the install
directory.
Set minimum required to 3.4 across the board. Use standard
GNUInstallDirs variables instead of custom INSTALL_BIN_DIR and
INSTALL_LIB_DIR. Use generator expressions and commands instead of
variables when possible. Use Runtime, Development, and Libraries
components in the installation. Set up the layout of the build tree
to match the installation tree. Replace compiler-specific flags with
more general cross-platform settings when possible. Update COMPILE
with new configure and install instructions. Add alias targets.
Checking them for being defined would get the user intention wrong if they are
passed as -DDISABLE_STATIC=Off as that would still count as being defined. Just
check for the variable, which has the desired effect. An unset variable is
always false. Since these flags are options now the variables are always set,
so one cannot disable this features anymore otherwise.
