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I have a separate development environment with privately compiled gcc and certain libraries. Lately I stumbled across an issue where CMake's PkgConfig uses the system-wide library, instead of from my private environment, though I think I have done everything according to the documentation.

The library in my case is x265. The experiment uses the latest snapshot of CMake (3.20.20210504-g576a070) from the repository. Let's assume my environment is at /home/SpaceX/gcc-env. The following paths are set in the system's environment:

PATH:
    /home/SpaceX/gcc-env/bin
    ...
LD_LIBRARY_PATH:
    /home/SpaceX/gcc-env/lib
LIBRARY_PATH:
    /home/SpaceX/gcc-env/lib
PKG_CONFIG_PATH:
    /home/SpaceX/gcc-env/lib/pkgconfig
C_INCLUDE_PATH:
    /home/SpaceX/gcc-env/include
CPLUS_INCLUDE_PATH:
    /home/SpaceX/gcc-env/include

pkg-config correctly finds information about x265:

$ pkg-config --variable=prefix x265
/home/SpaceX/gcc-env

Let's consider the following CMakeLists.txt:

cmake_minimum_required(VERSION 3.20)

project(mwe)

add_executable(mwe mwe.cpp)

find_package(PkgConfig REQUIRED)
pkg_check_modules(X265 REQUIRED IMPORTED_TARGET x265)

target_include_directories(mwe PRIVATE
    ${X265_INCLUDE_DIRS}
)
target_compile_options(mwe PRIVATE
    ${X265_CFLAGS_OTHER}
)
target_link_libraries(mwe
    PkgConfig::X265
)
target_link_options(mwe PRIVATE
    ${X265_LDFLAGS_OTHER}
)
message(STATUS "X265_FOUND = ${X265_FOUND}")
message(STATUS "X265_VERSION = ${X265_VERSION}")
message(STATUS "X265_PREFIX = ${X265_PREFIX}")
message(STATUS "X265_INCLUDEDIR = ${X265_INCLUDEDIR}")
message(STATUS "X265_LIBDIR = ${X265_LIBDIR}")

The following command

cmake -DCMAKE_BUILD_TYPE=Debug \
      -DCMAKE_C_COMPILER=gcc \
      -DCMAKE_CXX_COMPILER=g++ \
      -H. -Bbuild -G Ninja

will output

$ cmake --version
cmake version 3.20.20210504-g576a070
$ cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ -H. -Bbuild -G Ninja
-- The C compiler identification is GNU 11.1.1
-- The CXX compiler identification is GNU 11.1.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /home/SpaceX/gcc-env/bin/gcc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /home/SpaceX/gcc-env/bin/g++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found PkgConfig: /usr/bin/pkg-config (found version "1.7.3") 
-- Checking for module 'x265'
--   Found x265, version 3.5
-- X265_FOUND = 1
-- X265_VERSION = 3.5
-- X265_PREFIX = /home/SpaceX/gcc-env
-- X265_INCLUDEDIR = /home/SpaceX/gcc-env/include
-- X265_LIBDIR = /home/SpaceX/gcc-env/lib
-- X265_MODULE_NAME = x265

So far everything is alright. However, the compilation produces an unexpected result:

$ cmake --build build --verbose --config Debug --target all
[1/2] /home/SpaceX/gcc-env/bin/g++   -g -MD -MT CMakeFiles/mwe.dir/mwe.cpp.o -MF CMakeFiles/mwe.dir/mwe.cpp.o.d -o CMakeFiles/mwe.dir/mwe.cpp.o -c ../mwe.cpp
[2/2] : && /home/SpaceX/gcc-env/bin/g++ -g  CMakeFiles/mwe.dir/mwe.cpp.o -o mwe  /usr/lib/libx265.so && :

I.e. the generated linker command for some reason uses a full path to the system library: /usr/lib/libx265.so. Not even simply libx265.so, but an exact path nobody asked about. The library in question in fact exists at the X265_LIBDIR:

$ ls -lA /home/SpaceX/gcc-env/lib/*x265*
-rw-r--r-- 1 SpaceX SpaceX 12359618 мая  3 21:43 /home/SpaceX/gcc-env/lib/libx265.a
lrwxrwxrwx 1 SpaceX SpaceX       14 мая  3 20:51 /home/SpaceX/gcc-env/lib/libx265.so -> libx265.so.200*
-rwxr-xr-x 1 SpaceX SpaceX  8022848 мая  3 21:43 /home/SpaceX/gcc-env/lib/libx265.so.200*

I attached the MWE with x265.pc for reference. mwe.tar.xz