For libbrotly installed in a custom location, link explicitly...

to libbrotlicommon because it will not be found implicitly

See also https://github.com/google/brotli/pull/740

configure.ac

@@ -39,6 +39,7 @@ m4_ifndef([VARNISH_PREREQ], AC_MSG_ERROR([Need varnish.m4 -- see README.rst]))
 
 PKG_CHECK_MODULES([BROTLIENC], [libbrotlienc])
 PKG_CHECK_MODULES([BROTLIDEC], [libbrotlidec])
+PKG_CHECK_MODULES([BROTLICOMMON], [libbrotlicommon])
 
 VARNISH_PREREQ([trunk])
 VARNISH_VMODS([brotli])

src/Makefile.am

 AUTOMAKE_OPTIONS = subdir-objects
 
 AM_CFLAGS = $(VARNISHAPI_CFLAGS) -Wall -Werror -Wextra -std=c99 \
-	@BROTLIENC_CFLAGS@ @BROTLIDEC_CFLAGS@
+	@BROTLIENC_CFLAGS@ @BROTLIDEC_CFLAGS@ @BROTLICOMMON_CFLAGS@
 AM_LDFLAGS = $(VARNISHAPI_LIBS) $(VMOD_LDFLAGS) -ldl
 
 vmod_LTLIBRARIES = libvmod_brotli.la
@@ -17,7 +17,7 @@ dist_man_MANS = vfp_brotli.3
 
 vfp_brotli.lo: $(nodist_libvmod_brotli_la_SOURCES)
 
-libvmod_brotli_la_LIBADD = @BROTLIENC_LIBS@ @BROTLIDEC_LIBS@
+libvmod_brotli_la_LIBADD = @BROTLIENC_LIBS@ @BROTLIDEC_LIBS@ @BROTLICOMMON_LIBS@
 
 vcc_if.h vmod_brotli.rst vmod_brotli.man.rst: vcc_if.c