Before Apache httpd-2.4.26 compilation, openssl has been compiled. 
I built both (openssl and apache) on Windows with nmake. 

Openssl is placed in srclib folder:
\httpd-2.4.26\srclib\openssl
its headers - here:
\httpd-2.4.26\srclib\openssl\inc32\openssl
    
Apache compilation failed just after abs.mak execution.
Have a look at these lines in abs.mak (~330):


!IF  "$(CFG)" == "abs - Win32 Release"

InputPath=../include\ap_release.h

"..\srclib\openssl\include\openssl\applink.c" : $(SOURCE) "$(INTDIR)" "$(OUTDIR)"
	<<tempfile.bat 
	@echo off 
	IF EXIST ..\srclib\openssl\ms\applink.c type ..\srclib\openssl\ms\applink.c > ..\srclib\openssl\include\openssl\applink.c
<< 


The file applink.c cannot be found in \srclib\openssl\include\openssl
(if openssl built on Windows, it's in inc32 folder).

The output of apache:
        cd ..\..
        cd modules\ssl
        "C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\BIN\nmake.exe" -
nologo -f mod_ssl.mak         CFG="mod_ssl - Win32 Release" RECURSE=0
        rc.exe /l 0x409 /fo".\Release\mod_ssl.res" /i "../../include" /i "../../
srclib/apr/include" /i "../../build\win32" /d "NDEBUG" /d BIN_NAME="mod_ssl.so"
/d LONG_NAME="proxy_ssl_module for Apache" ..\..\build\win32\httpd.rc
Microsoft (R) Windows (R) Resource Compiler Version 6.1.7600.16385
Copyright (C) Microsoft Corporation.  All rights reserved.

        cl.exe @C:\Users\USER_NAME\AppData\Local\Temp\82\nmCEF9.tmp
mod_ssl.c
ssl_engine_config.c
ssl_engine_init.c
ssl_engine_io.c
ssl_engine_kernel.c
ssl_engine_kernel.c(2373) : warning C4018: '<' : signed/unsigned mismatch
ssl_engine_log.c
ssl_engine_mutex.c
ssl_engine_pphrase.c
ssl_engine_rand.c
ssl_engine_vars.c
ssl_engine_ocsp.c
ssl_util_ocsp.c
ssl_scache.c
ssl_util_stapling.c
ssl_util.c
ssl_util_ssl.c
Generating Code...
        link.exe @C:\Users\USER_NAME\AppData\Local\Temp\82\nm288.tmp
   Creating library .\Release\mod_ssl.lib and object .\Release\mod_ssl.exp
        if exist .\Release\mod_ssl.so.manifest mt.exe -manifest .\Release\mod_ss
l.so.manifest -outputresource:.\Release\mod_ssl.so;2
        echo Helper for Post-build step > ".\Release\postbld.dep"
        cd ..\..
        cd support
        "C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\BIN\nmake.exe" -
nologo -f abs.mak             CFG="abs - Win32 Release" RECURSE=0
        tempfile.bat
The system cannot find the path specified.
NMAKE : fatal error U1077: '.\tempfile.bat' : return code '0x1'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 10.0
\VC\BIN\nmake.exe"' : return code '0x2'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 10.0
\VC\BIN\nmake.exe"' : return code '0x2'
Stop.


The issue has gone after I modified abs.mak manually and replaced "\srclib\openssl\include" with "\srclib\openssl\inc32".
Please note that the issue does not occur with apache-2.4.20.