I had some issues building the lib inside a docker container in the form of
/src/pdf2htmlEX/../fontforge/inc/ffglib.h:33:10: fatal error: gio/gio.h: No such file or directory 33 | #include <gio/gio.h>

Added the following to CMakeLists.txt which fixed the issue:

Replaced the following
find_package(PkgConfig)

with:

find_package(PkgConfig)
pkg_check_modules(GLIB REQUIRED glib-2.0)
include_directories(${GLIB_INCLUDE_DIRS})
link_directories(${GLIB_LIBRARY_DIRS})
add_definitions(${GLIB_CFLAGS_OTHER})