交叉编译若使用pkg-config寻找package，需要设置一些环境让pkg-config能够找到交叉编译的库。
问题
若不设置，pkg-config会去默认的路径下寻找库，这样会导致找不到交叉编译的库。会出现类似以下错误：
CMake Error at /usr/share/cmake-3.16/Modules/FindPkgConfig.cmake:463 (message):
A required package was not found
Call Stack (most recent call first):
/usr/share/cmake-3.16/Modules/FindPkgConfig.cmake:643 (_pkg_check_modules_internal)
CMakeLists.txt:10 (pkg_check_modules)
从错误信息可以看出，CMake在调用pkg_check_modules时，pkg-config找不到对应的库。
解决方法
设置几个环境变量
# sysroot是所有交叉编译文件所在的根目录，具体参考 [cmake sysroot](https://cmake.org/cmake/help/latest/variable/CMAKE_SYSROOT.html)
export SYSROOT=/path/to/sysroot
# 以下的路径自己查看sysroot下到底有没有，填充的思路就是找到pkgconfig目录填进去即可。
export PKG_CONFIG_PATH=
export PKG_CONFIG_LIBDIR=${SYSROOT}/usr/lib/pkgconfig:${SYSROOT}/usr/lib/aarch64-linux-gnu/pkgconfig
export PKG_CONFIG_SYSROOT_DIR=${SYSROOT}
为什么要设置这些环境变量
参看pkg-config的 man page 节选：
ENVIRONMENT VARIABLES
PKG_CONFIG_PATH
A colon-separated (on Windows, semicolon-separated) list of directories to search for .pc files. The default directory will always be searched after searching the path; the default is
libdir/pkgconfig:datadir/pkgconfig where libdir is the libdir for pkg-config and datadir is the datadir for pkg-config when it was installed.
PKG_CONFIG_SYSROOT_DIR
Modify -I and -L to use the directories located in target sysroot. this option is useful when cross-compiling packages that use pkg-config to determine CFLAGS and LDFLAGS. -I and -L are
modified to point to the new system root. this means that a -I/usr/include/libfoo will become -I/var/target/usr/include/libfoo with a PKG_CONFIG_SYSROOT_DIR equal to /var/target (same
rule apply to -L)
PKG_CONFIG_LIBDIR
Replaces the default pkg-config search directory, usually /usr/lib/pkgconfig:/usr/share/pkgconfig.
...
- PKG_CONFIG_PATH 是pkg-config寻找.pc文件的路径，设置这个变量可以让pkg-config在指定的路径下寻找.pc文件。
- PKG_CONFIG_SYSROOT_DIR 是用来修改-I和-L的，让pkg-config在交叉编译时能够找到正确的头文件和库文件。
- PKG_CONFIG_LIBDIR 替代默认的pkg-config搜索路径，这个变量可以让pkg-config在指定的路径下寻找.pc文件。