Feature/allow nondefault install prefix

[alex-che]

These changes allow curlpp to be installed to non-default location.
E.g., by using CMAKE_INSTALL_PREFIX configuration variable.
Currently, if I use non-default install prefix for curlpp, then curlpp examples could not be build.
With the proposed changes, I could build curlpp examples using PKG_CONFIG_PATH or CMAKE_PREFIX_PATH pointing to the directory containing curlpp.pc file.
The changes should also fix #68 .

[XMNXofficial]

sorry,it still error

[main] Building folder: examples 
[main] Configuring project: examples 
[proc] Executing command: "C:\Program Files\CMake\bin\cmake.EXE" --no-warn-unused-cli -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=TRUE -DCMAKE_BUILD_TYPE:STRING=Debug -DCMAKE_C_COMPILER:FILEPATH=C:\Windows\w64devkit\bin\gcc.exe -DCMAKE_CXX_COMPILER:FILEPATH=C:\Windows\w64devkit\bin\g++.exe -SC:/curlpp-0.8.1/examples -Bc:/curlpp-0.8.1/examples/build -G "Unix Makefiles"
[cmake] Not searching for unused variables given on the command line.
[cmake] CMake Warning (dev) in CMakeLists.txt:
[cmake]   No project() command is present.  The top-level CMakeLists.txt file must
[cmake]   contain a literal, direct call to the project() command.  Add a line of
[cmake]   code such as
[cmake] 
[cmake]     project(ProjectName)
[cmake] 
[cmake]   near the top of the file, but after cmake_minimum_required().
[cmake] 
[cmake]   CMake is pretending there is a "project(Project)" command on the first
[cmake]   line.
[cmake] This warning is for project developers.  Use -Wno-dev to suppress it.
[cmake] 
[cmake] CMake Warning (dev) in CMakeLists.txt:
[cmake]   cmake_minimum_required() should be called prior to this top-level project()
[cmake]   call.  Please see the cmake-commands(7) manual for usage documentation of
[cmake]   both commands.
[cmake] This warning is for project developers.  Use -Wno-dev to suppress it.
[cmake] 
[cmake] -- Checking for module 'curlpp'
[cmake] --   
[cmake] CMake Error at C:/Program Files/CMake/share/cmake-3.26/Modules/FindPkgConfig.cmake:607 (message):
[cmake]   A required package was not found
[cmake] Call Stack (most recent call first):
[cmake]   C:/Program Files/CMake/share/cmake-3.26/Modules/FindPkgConfig.cmake:829 (_pkg_check_modules_internal)
[cmake]   CMakeLists.txt:3 (pkg_check_modules)
[cmake] 
[cmake] 
[cmake] -- Configuring incomplete, errors occurred!
[proc] The command: "C:\Program Files\CMake\bin\cmake.EXE" --no-warn-unused-cli -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=TRUE -DCMAKE_BUILD_TYPE:STRING=Debug -DCMAKE_C_COMPILER:FILEPATH=C:\Windows\w64devkit\bin\gcc.exe -DCMAKE_CXX_COMPILER:FILEPATH=C:\Windows\w64devkit\bin\g++.exe -SC:/curlpp-0.8.1/examples -Bc:/curlpp-0.8.1/examples/build -G "Unix Makefiles" exited with code: 1