Optionally build Crow as a library

[bugdea1er]

I think it would be useful for Crow to be able to be compiled as a library (static or shared). This would reduce build times for clients and allow the Crow distribution to be updated separately (in the case of a shared library)

Note that in this case it is still possible to compile everything into crow_all.h

[SimonCahill]

Crow can already be used as an INTERFACE library in CMake projects.

Here's an example of how I do it, using CPM:

SET(CROW_BUILD_FUZZER OFF CACHE BOOL "Build the fuzzer for Crow")
SET(CROW_ENABLE_SSL ON CACHE BOOL "Enable Crow's SSL feature for supporting https")
SET(CROW_ENABLE_COMPRESSION ON CACHE BOOL "Enable Crow's Compression feature for supporting compressed http content")
set(CROW_BUILD_EXAMPLES OFF CACHE BOOL "Build the examples in the project")
set(CROW_BUILD_TESTS OFF CACHE BOOL "Build the tests in the project")

# some magic here which fetches the info
CPMAddPackage(
    NAME ${PACKAGE_NAME}
    GIT_REPOSITORY ${PACKAGE_REPO}
    GIT_TAG ${PACKAGE_TAG}
)

list(APPEND ${PROJECT_NAME}_CPM_TARGETS ${PACKAGE_NAME})

# ...

target_link_libraries(${PROJECT_NAME}
    ${${PROJECT_NAME}_CPM_TARGETS}
)

Then you can just:

#include <crow.h>
#include <crow/middlewares/utf-8.h>

Building Crow as a static library doesn't make sense, because it's header-only.